This is typical of having problems with your network. Almost any network can have random, but very rare disconnects, but if they are occurring frequently, I'd have a technician check your network for problems. If your computers are wired to the network instead of using WiFi, it could be as simple as a bad Network Interface Card or a bad patch cord connecting the computer to the LAN.
Thanks for reply. I checked the network. All are ok no problem. I put the server setting on Energy save "hard disk never go to sleep" and the client the same " hard disk never go to sleep". but still the database keeps closing. when we create a new entry or change a record after closing the connection we lose the data.
Welcome to the world of endless frustration that FileMaker Server is. Failed connections are a feature of this product. At least with FMS 13 we escaped the Java console plague, but that's about it. And don't get me started on ODBC just yet.
Get this: it is way more reliable AND FAST to connect to your server with VNC or other remote desktop client controlling a client on the server than it is natively with a local FMP accessing the remote database.