Unable to access WordPress admin panel

The reason for the error should be visible in the error log. You can find it in your hosting area. If necessary, the support of your hosting can help you.

Alternatively, you could also enable debugging in WordPress. How to do this is described here: https://ww.wp.xz.cn/support/article/debugging-in-wordpress/ – also here the support of your hosting can help you if necessary to edit the file accordingly.

In both cases you should be able to see what the cause is in the logfile.

The message is abbreviated and thus literally says nothing at all. The best thing to do is to contact your hoster’s support.

The mentioned file wp-content/db.php is not part of WordPress Core. You would have to find out, as written at Stackoverflow, where this file comes from. Mostly plugins put files there – or they get there via a hack through a security hole in a plugin or theme. Since you still can’t see the complete error via screenshot, you can’t say more about it from here.

Theoretically yes. But if the file is included by something (which must happen since it causes an error), then you paralyze your website with the deletion. You would only get an error that a file was not found. Therefore, the search for the origin is quite important. Hints for this could also be in the file.

It says so right above: this obviously belongs to the plugin W3TotalCache. Deactivate the plugin to solve the error message. If you have any questions, please contact their support forum.