Does it open if you play with the handle in various positions? It sounds like the mechanism is slightly out of adjustment. If you do manage to get it open try tweaking the adjustment screw a half turn at a time. That can be seen once the window is open in normal mode.
Tilt and turns are a nightmare, you will need to open the window fully and then find the little latch that operates the gearing and hold it in while lifting the handle to put it in the tilt postion while doing this get someone to check all the bolts are moving( i bet the aint) if thats the case it sounds like something has snapped or where the gearings join one of them have popped out
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