You'll need to investigate things and report back in more detail.
Are you using Open Remote to open this hosted file?
Are you able to use the admin console to check on this file on your server to see if it is on the server and open?
Do you have back up copies of your file? (Server should be set up to create at least daily back up copies of every file that it hosts)
There are a lot of possibilities. The first would be to confirm that the network is good from your client to the server. Maybe you can ping the server or otherwise confirm that you can see the server. I guess you can also do basics like confirm your server is turned on and that it is on the network too (e.g., screen share into and maybe open a web page to confirm the network is working?). Can any other computer see the server and open FileMaker? Have you opened the Admin Console and confirmed in the Activity tab that the Database is open and being served? Start with checking those things first and we'll help out.