It's almost certainly a database issue. Contact your host, as Andrew suggested. If you feel comfortable doing so, you can try checking for database errors in phpMyAdmin.
First, back up your WordPress database before trying anything. If you have access to cpanel, there's most likely a simple option to create an SQL backup.
To check for/repair errors, log in to phpMyAdmin, then:
- Choose your WordPress database and select the "Structure" tab.
- Check the box next to every table listed (You can just click on Check All ).
- Select "Check Table" to search for errors.
- Select "Repair Table" to repair any errors.
Note: Your site may be compromised. However, I do see this problem occasionally when a host automatically upgrades WordPress to a new version.
It has usually shown the "5 minute install" screen for a few hours until I refresh /wp-admin/ and it's suddenly back to normal. So, if the problem goes away on its own, you probably don't need to do anything. Otherwise, the steps above should fix the problem. Either way, I would read up on "hardening WordPress". Good luck!