To be honest, I've never heard of anyone's password "expiring."
Of course, this is the first time I've heard of a former employee keeping a laptop with full disk encryption installed.
Presumably during this time, it was unable to "phone home" to the Endpoint management server to receive updates, etc.
A screenshot of what you see when you attempt to log in might be helpful, but I suspect you're out of luck.
The only way to get into the laptop would be:
- Logging in with a different account that is authorized to so
- Decrypt the hard drive using the recovery file
I'm guessing you have neither of these files and, given it's been a couple years, your former employer no longer has this information either.