There are a few possible causes for this issue. First, you should check to see if the backup process was completed successfully. If it did, then you may have an issue with the server or the configuration of the site.
If the backup process was not completed successfully, it could be a server issue or a Magento issue. Either way, you should contact your web hosting provider and ask them to investigate the issue. They should be able to help you determine what is causing the issue and how to fix it.
If you are having trouble accessing the Magento admin, you may need to reset your admin password. To do this, follow the instructions in the Magento User Guide.
Finally, if the issue persists, you may need to restore your site from the backup. To do this, you will need to contact your web hosting provider and ask them to help restore the site from the backup.