I have Joomla 3.3.6 and K2 version 2.6.8.
Until recently I made a lot of articles with K2 and it worked perfectly. But yesterday i wanted to make a new article via joomla backend but it did not work anymore. When I clicked on the items or categories button in the joomla control panel nothing happens and i got a message that there is a connection problem. There is however no connection problem because all the other extensions in joomla work perfectly.

Does anyone know what causes this problem.

Thanks in advance

Could you try repairing your DB tables?
If you do not have access to your database, an extension called Admin Tools can do this for you.

PS. Backup first.