Your backup file is usually stored in your browser’s cache - this is why whenever you’re logged out, you only need to enter your password to regain access to your wallet. Clearing your cache logs you out completely.
It’s no cause for worry, however. You can simply restore your wallet using your backup file.
Go to https://guarda.co/app and select Import or Restore.
Then click on Restore from Backup.
Either drag and drop the .txt file with your backup into the specified field, find it in your files via Browse, or enter the code manually.
Then enter your password. Welcome back to Guarda!