Thank you for posting.
Assuming that nothing has changed on the server, it's possible that the installation or databases are corrupt. To check that the databases aren't the issue, simply move all of the databases out of the installation directory's Databases folder and try hosting a test database instead.
If you still get the error, you may want to try a reinstallation of FileMaker Server. It would be safest to first install and host your files on a test machine if you have one available. This will also help determine if something else on the original machine is causing the issue. If you suspect this is the case, you can simply try disabling all non-essential startup programs.
Also check whether there were any recent java or other system updates. Any information you can provide about the OS or system specifications would help.