To add to what TE said, I have never seen a 5000 system just "lose" it's license for no reason. If that happens it is almost guaranteed to be a CF card or other hardware issue which would require more than the just a license file.
Best practice would be to maintain regular database backups, a copy of the software (admin and system) that your system is running, and the license file... The later two items can only be obtained by a Mitel dealer, and TBH are probably the least important because if you need to replace hardware like the CF card, they are serialized and the license file will need to be regenerated anyway.