I recently upgraded my Joomla website to 2.5.x using the JUpgrade component.

Now, all my K2 items are not showing. The only way to make them show is to open them in the backend, one by one, and just save them without doing anything else.

FYI, I have already tried the UPDATE query regarding the access in the database.

I also tried to see anything suspicious in the k2 items table but I can't notice any difference between the showing items and the ones that don't show.

use phpmyadmin to check the access of the category should be 1

Categories' access is also set to 1

@Manos Krokos

The language field should set to "*" instead of being blank.

Yes, that did the trick, Lefteri! Thank you so much!