I know the feel, sometimes you have ideas bouncing in your head but when you sit down for some reason they either don't come out, or you just can't write. For me, I've always done several things to try to get me into a mood where ideas can solidify:
i) Take a long bath/shower (always works for me)
ii) Take your smart phone or notepad and go out for a walk, or just do anything but stare at your computer screen, and go fantasize about your story.
iii) Screw up your sleeping schedule big time by sleeping really, really, reallyyyy late (I mean like 3, 4, 5, 6am).
iv) Take drugs. (no I'm just kidding)
Anyways these things work for me, so I'm not sure if they will for you. Good luck!