So to re-focus on the issue ... you cannot get to any of the Site Admin menu links that go to admin areas, is that correct?
Above would mean one cannot click through them to purge caches but can you go directly there via the URL line in your browser?
Example: https://yoursite/admin/purgecaches.php
You change https://yoursite/ above to the URL of your site.
If you are not logged onto the site as an admin, you should be prompted to login and after that Moodle should forward you to the admin/purgecaches.php script. In your version of Moodle that script is supposed to just do purges ... you will not see options form to choose what to purge.
One can do manual purges via cPanel ... and Visvanath Ratnaweera has already listed the directories you should purge: cache, localcache, and muc. Those should be rebuld as site is used.
You should NOT do any more with folders and files in moodledata ... just the above.
If that doesn't clear up issues, then you need to turn on debugging ...
First see if you can get to the form for debugging directly:
https://yoursite/admin/settings.php?section=debugging
If you can set debugging to developer and then try to navigate to any other admin area. If debugging shows something copy and paste to a local text document (like NotePad/TextEdit) ... then come back here and share what that said.
After you've done that, turn debugging off by going back to the debugging URL and turning it off.
IF you can't get to debugging that way, there is one alternative left and we can provide that when/if needed.
Without some 'techie info', anything would be a total guess and may not be anywhere close to resolving your issue.
It also wouldn't hurt to tell us something about where and how you are hosted ... hosting provider? ... shared hosting? plan you lease? etc.
'SoS', Ken