Thanks for all your replies guys!
There a miracle has happened

, hehe. I dismantled the nozzle, rummaged a little in it, and turned a gun on once again, waited it to reheat and tadam! it works now.
But anyway, I went to Castorama today (after I went mushroom picking :D) and bought a glue thinner. Just for a nozzle and its nut, hope it works.
@Miguel - why shouldn't I use a glue gun for putting foam together? It's very comfortable...
@Pam - here in Poland, glue guns aren't somehow particularly expensive, so if my gun wouldn't work anymore there was no problem to buy another one. But if I lived next to you, I wouldn't deny to receive one of your spare ones, of course.

Iza
