macOS Big Sur update problem

I recently updated my MacBook Pro 13" 2015 to macOS Big Sur and now it's not working! Did they stop supporting older laptops? Maybe there's something I can do to go back to Catalina or Mojave?

Or at least recover the data from the disk. I'm working on a website and there's plenty of vital info on that disk, if I loose it my project would suffer a lot.

Thanks in advance. I asked this question on other forums but didn't get the answer...

No, the Big Sur support for older hardware is the same now as it was in June when it was publicly released. The only temporary change is two models are temporarily blocked from starting an update - https://mrmacintosh.com/big-sur-11-0-1-20b50-released-to-block-install-for-2013-14-13-mbpros/ but your model should be good to go for restores and installations.

What does and now it's not working! mean precisely?

Now, older hardware goes out of support all the time based on the age of purchase and has nothing to do with whether the gear runs the new OS. Either way, if you want to ask a follow on question for your specific symptom, we may have help for that whether it’s out of “official support or not, but hopefully this answers how Apple will support you.