unfortunately, when FileMaker says this, it's true in most cases...
- is the correct server/host selected?
- are other files from that server/host available?
can be a problem with the network (network address changed..), the host can be down - lot more
Are You on a local network (server is somewhere at Your location)?
The error means what it says. Your FileMaker client is attempting to access a hosted file that is not currently accessible.
This could be a network issue.
The server might not currently be running
The file could truly no longer be on that server.
The file could be on the server, but closed
You'd need to tell us more about this file and its server before we can help you more.