TOMATO ADVICE BLOG'S TROUBLESHOOTING GUIDE TO "DID I KILL THIS HOUSEPLANT?" BASED ON MY TIME AS A RETAIL GREENHOUSE WORKER:
1. Look up if it's an annual, biannual or perennial first. Most houseplants are (supposed to be) perennials, but there are a few annuals/bianuals that get sold as houseplants. Amaryllis can survive for many years if properly potted and you have the mandate of heaven on your side, but they are annuals. That said, even an annual should live at least 5-6 months at minimum.
2. If it died very suddenly (like "looked kinda sad one day, worse the next and wholly deceased on day 3") a couple weeks after you got it? Not your fault. It was already infected with a greenhouse fungus when you got it, and there was nothing you could do for it.
Greenhouse fungi are extremely common and effect the majority of retail plants sold in the US: they spread virulently through greenhouses AND can infect any plants transported with them, and plants transported in the same truck afterwards. Outdoor plants have more resilience against them because other microrhyzal fungal colonies in their outdoor pots or garden beds will protect the plants, but houseplants are kind of screwed.
You can take a swing at mitigating this by immediately repotting any houseplants you receive with dirt from a pot/garden bed/part of the yard that has other plants actively growing in it (remove.other plants before putting that dirt in with your houseplant), and putting your houseplants outside when the weather is warm, but it's often a lot cause by the time you receive the plant.
Greenhouse fungi infections are the #1 killer of retail houseplants in my experience.
3. If your plant dies EXTREMELY suddenly, like "fine last night and dead this morning" something in the building it's kept in poisoned it.
Likely culprits: cats peeing in the dirt, small children pouring soda in there (sugar aggravates any infection it might have), shitty coworkers pouring coffee in there, and accidentally hitting it with a cleaning spray while you were sanitizing the kitchen counters.
4. If it dies very slowly over the course of a couple of weeks within a year of you getting it, I'm afraid you probably killed it. The two main ways people kill houseplants are
A) Over Watering. How to fix it: keep your plant pots in a large, high-sided, no- drainage container like a large Tupperware or boot tray. Once a week (twice when AND ONLY WHEN it gets to be +80 farenheit in the room where the plant lives) fill the container with an inch or two of water, and let the plants absorb it through the bottoms of their pots, AND DO NOT DEVIATE FROM THIS SCHEDULE. If you must deviate, err on the side of under-watering them, that's a lot easier for a plant to recover from.
B) Not Enough Light. Most houseplants are tropical understory plants because those are the only ones that will tolerate the "Total Shade" level of ambient light in most houses. Succulents, cacti and most woody houseplants are not understory plants. They need 8-12 hours full spectrum light, and most glass that windows are made of block a large part of the spectrum they need. Get some grow lights. You can use the purple ones as fun night lights for your house that won't mess up your vision or sleep cycle when you get up in the middle of the night!
C) Not "common" but often enough: over-feeding. Potting soil does not need that much amending, and adding plant food to fresh potting soil will scorch the roots. Don't.
5. If your perennial plant that was thriving suddenly dies after three years, ESPECIALLY if it was an orchid: not your fault! The way that many greenhouse plants are grown is FUCKED.
Orchids in particular are doomed: orchids are heavily specialized and extremely dependent on microrhyzal fungi to stay alive. Like, parasitically dependent. As in, orchids make literally millions of microscopic seeds in hopes that one will land somewhere that has the extremely specific species of tropical fungus that orchid can hack to stay alive. Because the orchid's fungal needs are so key and so specific, greenhouse orchids are grown in a way that dooms them to tragically brief lifespans.
Greenhouse orchids are grown in sterile conditions by placing the seeds in agar and pumping them full of growth hormones and food tailored to that species exact needs (that's why there's only a couple dozen commercially sold orchids of the tens of thousands of species in the family), and continue pumping them full of their specific super food until they're large enough to be sold, and they're usually sold with Orchid Food.
Imagine growing a baby in a test tube, but the baby's immune system comes from bacteria it would be exposed to in uetero, so your lab baby has no immune system, so you feed it shitloads of vitamins to prop it up against infections. How long do you think that baby would survive outside of the lab, even if it's keeper kept up the vitamin regimen?
In the case of most orchids, about three years.
You CAN make an attempt to save your doomed bubble baby. You can go outside, find SEVERAL places full of vigorous and lively plants, pull up one of those plants (preferably one that doesn't regerminate from severed roots, like thistles) knock the handful of dirt that comes up with it into your collection of Very Alive Plant Dirts, and repot your orchid in a well-drained pot with that mixture and some orchid soil. IF YOU ARE EXTREMELY LUCKY, there will be a microrhyzal fungus in your wild dirt samples that is close enough to your orchid's host species that it will be able to accept it as it's new immune system. This is literally a one-in-a-million shot, but I *have* seen it work, and the rescued orchids live for DECADES.