Error message: "Please enter passphrase for disk Virtual Disk error"
This message would normally occur if there has been a change in the machine's UUID. The FMOS software encrypts the disk using the machine UUID, and if this changes, it constitutes a change of hardware, which is seen as a security issue by the LUKS disk encryption.
Unfortunately, there is no way to recover from this except to rebuild the FireMon server and restore from backup.
If you have moved the VM between VM hosts, then you need to move the FMOS VM back to the host it was installed on, then it will boot correctly.