"Device not ready" usually means the server lost a disk (if your disk is a SAN device), check with your SAN admin to see what happened.

You can confirm it by looking in the windows system event log, you should see messages from the SAN vendor driver (i.e. EMC drivers complaining a path/port/device just died).

Once you get your disk back (after a few seconds or after intervention) you can try: ALTER DATABASE [MyDB] SET ONLINE

If that fail, the error log will give you more info on what's going on.