SSH key randomly stopped working -- user@host.com: Permission denied (publickey).

Okay, it's sorted. I had accidentally deleted my home directory last session (I wasn't using it for anything so it wasn't an obvious mistake) and so with it the .ssh folder disappeared, wiping the authorised keys file. Restoring the file with my SSH public key resolved the issue. Sorry for wasting your time, but hopefully this will serve to help anyone else who ever has this issue.