How to fix 3.5mm jack?

Pawel Shmeliow, the first thing I'd try is to see if there is any debris inside the jack. Invert your iPod and use some "air in a can" electronic duster and blow it carefully inside the jack. I prefer to not stick anything, like tooth picks or cotton swabs, in there. Now thake your headphones. and insert them all the way and remove them quickly. Try that a couple of times to make sure the contacts on the inside are not stuck. If none of that works, it may be time to replace the jack. This video shows you how to do it. The part is available at places like this. Hope this helps, good luck.