|>>|| No. 3025
I think you need the www in there for the thumbnail:
If you can, it might help to take the curtain off and record what's going on at the hinges from both sides, taking care not to drop your phone out of the window.
It's not terribly uncommon for the tilting and locking mechanisms of these things to get stuck. Sometimes it's just a matter of jiggling the bastard around until something moves correctly. You might try gently lifting the window up vertically, so the weight of it is off the hinges, then moving it back and forth to see if anything pops back into place.
It could also be a proper locking mechanism intended to keep the window open. Look around the frame for a sticker or any brand name / model number printed onto it, then do a Google search on it. You might get lucky and find an instruction manual, then you can see if it's actually intended to lock open or not.
Again, I emphasise, don't fall out of the fucking thing.