WE G18 issue

[NathyB]

So I have a g18 and the fire selector is stuck on single but shoots full auto and the selector won't move and it's very difficult to release the slide

 

I was just messing about with the gun cocking it and dry firing it, then the slide wouldn't return all the way like 95% there my friend stripped then gun and every thing is fine nothing is visibly broken has any one has this issue ? And know how to fix it thanks