If you physically lose the key, then you must assume that somebody else has it. Therefore, the key would be considered compromised and you should indicate that in the revocation information.
You would use "key is no longer used" if you retired the application that was using it, or if you decided to switch to a different key/program/scheme. Since all revocation certificates do the same thing (make your key no longer valid), it's not a big deal if you don't include the specific reason for revocation. Also, if you're retiring the key gracefully, you can just make the certificate right then.
It is, of course, really important that you always have access to a revocation certificate in case the key needs to be invalidated.