You log into your WordPress admin Dashboard and all of a sudden the CSS is broken, uh oh. Fear not, this is definitely not an unheard of problem. Fortunately, there are several different possible solutions to fix this particular CSS issue.
The first step is to identify if you recently installed any plugins or themes as there may be a conflict or bug somewhere that needs sorted out. If this is the case try to disable/deactivate the plugin or theme you recently installed. If you are unable to navigate the admin Dashboard due to the CSS issue, you can also go to the plugin or theme folder through FTP on the web server and rename it (this will also disable it). If this solves your problem then there is an incompatibility issue with the plugin or theme you are running.
If you didn’t install a plugin or theme recently, or the first step did not solve the problem, the next step is to try a fresh install of WordPress. You can either do this through FTP by overwriting the WordPress base files, or by clicking the reinstall button under the Updates section in the WordPress Dashboard.
The next option if you still do not have a solution is to turn off WordPress’ default concatenate scripts constant by adding this line to your wp-config.php file:
Finally, if you still do not have a solution to this issue it is time to turn WordPress debugging on by adding the following line to the wp-config.php file:
After adding this you should start seeing errors appearing on the affected pages which will give you clues as to the issue. You can also use the Chrome or Firefox Inspector to assist with your debugging. It is best to leave this option to a professional with experience in web development as this can be a tedious process.