The following are steps to troubleshoot a default gateway not available error:

1. Check that the Ethernet cord is securely plugged into the router and computer.
2. Verify that the router is turned on.
3. Check if any network or wireless settings were recently changed on the computer.
4. Restart your computer and reconnect to the router.
5. Check the firewall settings of your computer.
6. Check if your IP address is valid.