Update can not see plugins

Hi,
first of all check the permisions of the files of the plugins(most likely this is not the case but it doesn’t take long).
If no issues there, download the plugins files via ftp, zip them and upload them throw wordpress backend this way if there are any issues with the plugin(any reason not to work with wp3.5.1) then the backend will throw an error.

Thanks, I think it may have been hacked.
Every php file has strange code added to the top.
<?php $zend_framework= [hacked code removed]

that goes on for about 100 lines)

If your site has been hacked, these should be helpful:

https://codex.www.ads-software.com/FAQ_My_site_was_hacked
https://www.ads-software.com/support/topic/268083#post-1065779
https://smackdown.blogsblogsblogs.com/2008/06/24/how-to-completely-clean-your-hacked-wordpress-installation/
https://ottopress.com/2009/hacked-wordpress-backdoors/

Additional Resources:
https://sitecheck.sucuri.net/scanner/
https://www.unmaskparasites.com/
https://blog.sucuri.net/2012/03/wordpress-understanding-its-true-vulnerability.html