glock 17 blowback slide stuck

[Tudor]

I've just bought a used glock 17 full metal, gas,  blowback from my friend and it s my first BB gun. I wanted to see what s inside so i took of the slide and the spring, and i think i put either of them back on wrong and i pressed the trigger. When i tried to reload (without gas) it didn t go back so i tried to slide it out again and it s stuck. I can t get it to go backwards at all but it can go forwards about 2 cm.