are the least of your problems... I would just assume there gone for now.
1 fish would not cause this problem. the bacteria will rebound it will just take a little time. Do not add any ne fish or inverts for a few months till things are running fine again.
I feel your pain, I went through a big crash about 2 years ago and the tank is started to come back about 1.5 years after the crash. Recovery can be slow so take things slow.