Which model do you own and how does the loss manifest?

The single-tap and long-tap don't produce expected output? Can you share more info on it?

I own many Yubikeys (due to research I've been doing in 2017.) and I had many Yubikeys to play with, for TOTP/HOTP/U2F purposes, even using it to unlock Windows and I haven't had a case of a Yubikey basically deprogram itself. I washed them in the washing machine, ran them over with my car, thew them in mud piles and they always worked without a fault so your case is a surprising one.

Judging by what you wrote, unless there's some weird NFC communication going on between your phone and Yubikey (are they in proximity?), I'd say it's faulty and you need a new one.

Classic USB. Plastic. (I don't know the exact model, I got it from work.)

After adding a site or a computer it works a few days and then suddenly when I try to use it with my phone or computer I just get an error about no <something>.

So yes, probably defective.

Not enough info to even guess what might be wrong, but I'd assume it's defective and I'd try with another key as well. I wish you good luck with the next Yubkey you get! :)

Btw. this is the first time I've read on a public forum that someones Yubikey is defective, they are really well made and I didn't manage to break one via regular use and bad maintenance.