If you're experiencing the "failed to find a lobby" error when opening Diep.io, clearing your cookies and cache might resolve the issue. Follow these steps to do so:

Guide for Chrome/Edge

1. Open Chrome/Edge and go to Diep.io.
2. Click on the lock icon located in the address bar at the top-left corner of your browser.
3. From the dropdown menu, select "Cookies & Site Data".
4. Click on "Manage on device".
5. Click on the trash can icon next to "diep.io" to remove the site's cookies and data.
6. Perform a hard refresh: Press CTRL + F5 to do a hard refresh of the page.

Guide for Firefox

1. Open Firefox and go to Diep.io.
2. Click on the lock icon located in the address bar at the top-left corner of your browser.
3. From the dropdown menu, select "Clear Cookies and Site Data...".
4. In the dialog box that appears, locate "diep.io".
5. Select "diep.io" and click on "Remove".
6. Perform a hard refresh: Press CTRL + F5 to do a hard refresh of the page.