I bought bitcoin years ago and now the website that I bought from is unavailable! I only have a key that I don't even know whether it's a public key or a private key or some useless key generated by the website. Plus I'm not sure that the website bought actual bitcoin or other cryptos for me. How can I know if I have it on the blockchain?
You can try to import the wallet using the key you have (assuming it is a private key) through wallet software such as bluewallet.io, which supports importing of wallets via private keys.
Otherwise, if that key is not your private key, you are a little screwed, sorry.