Is there any way to find out who removed the password?
not from a FileMaker perspective.
How do I protect this from happing again.
Dont let anyone in your system unless they are trustworthy and, if its a company, do they have guarantee, warranty, or insurance in case there support guys do damage?
And to add to coherentkris' advice: enable Encryption-at-Rest (EAR) on your FM file. That way nobody can use any of the existing tools to remove a pw. Also make sure your file is not set up to auto-login obviously.
"Passwords was removed from a file"? What does that mean? What evidence are you seeing? Could it be that you were expecting the password to be auto-entered and now it's no longer saved? What version of FileMaker are you using.
For a non-FileMaker person, removing a password is a bit involved. Getting into the file with a full-access account, modifying the security, while not unsurmountable it isn't trivial either. Even for a FM developer, you still need to know certain things to get in. Assuming FM 15.
Could it be the password was saved in the computer's credential tool (OS X Keychain, etc.) and it is no longer there?