1 – Start with the basics first.
Have you tried:
– deactivating all plugins to see if this resolves the problem. If this works, re-activate the plugins one by one until you find the problematic plugin(s).
– switching to the Twenty Twelve theme to rule out any theme-specific problems.
– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems.
– re-uploading all files & folders – except the wp-content folder – from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones.
2 – Give the 3.5 Master List a read:
https://www.ads-software.com/support/topic/troubleshooting-wordpress-35-master-list?replies=4
> Insert Media button doesnt work
The Insert Media button can be a javascript issue and you can confirm that with by enabling the javascript console on your browser.
https://codex.www.ads-software.com/Using_Your_Browser_to_Diagnose_JavaScript_Errors
> Thumbnail Images no longer work
Can you provide us with a link to your site?
