Mac or Win? Version?
What programs are running at startup? The server may be running at startup.
Mac - Activity Monitor in Applications > Utilities
Win - Task Manager (right click on systray)
It be an error or corruption. Do you do monthly maintenance?
Mac - AppleJack
Win - ccleaner and defraggler
Has the file been set up to be shared?
Open the file with filemaker and check Files | Sharing.
I'd give a more detailed description, but I don't have that version installed and the details may differ.