in SQL Server Management Studio, connect to the dabase server and make sure you are using the correct database.
Then
select SettingValue from HostSettings where SettingName = 'ControlPanel'
See what it says. Mine says "Admin/ControlPanel/RibbonBar.ascx"
If yours is different, you can check the location (physical file path in your DNN install) and see if the ascx file is where your database thinks it is. If not, but the RibbonBar is where it should be according to my output, then you can
Update Hostsettings
set SettingValue = 'Admin/ControlPanel/RibbonBar.ascx' where SettingName = 'ControlPanel'
Obviously you want to proceed with caution. I often precede such updates with a "begin transaction" and execute that, and then run the update statement. You should get a message that 1 record was updated. If that isn't the case, then I'd do some selecting to find out what all happened, and be ready to do a "rollback transaction" which puts things as they were. But really, you should have no issues, and 1 record should be updated, and when you do a select, it should look fine. So then you need to close out the transaction with a "commit transaction" statement. Execute that. Then go back to your site and see if you are able to log in.