I tried to repair tables from joomla and from adminphp. I tried to reinstall k2 over the old one but nothing.i saved the database and the files and after the update i put all the old ones back.

Note that i did everything i could think.

Can you remember any operations you performed?

Nothing. The one day was ok and the other wasn't. The previous day i added an article and the other day when i couldn't receiving that specific error

Any changes in PHP version/ server setup?

Nothing. I asked my hosting and they said they didn't changed anything

Try the following,

Disable all extensions one by one to see which if this is caused by a 3rd party extension.