I get stuff like this all the time and i think to cure it i need a good teacher, though it could be tiredness.
It's usually the little things that i forget and thus can't play well, or i just can't get the feel right for some reason, usually because i've just been playing CoD 4 or something or my hands are cold. Lately i can not play shreddy speedy fast stuff very well at all, BUT my riff playing and general riffing abilities seem to be growing, which is what i'm after.
After years or wanting to play real fast, it's the last thing i want to do, now i want to actually make music and cool riffs and have fun, it's way more fun and people actually want to hear that kind of stuff and hey, can even get a band together and make songs! play gigs! who would've thought eh?
I think I play bass in bands because my guitar abilities have been stuck in trying to be a monster of a soloist, which i never will be. Though i do love playing bass and i often come up with stuff on bass, then transfer it onto guitar.
(cr@p can i type fast on this keyboard!) anyway, i think the only advice i can give is to just stop, let your mind work over on it and maybe you just are fatigued in some way, or stressed?
I love how our minds work things out for us, one day we can't get it, after we leave it a lone and our minds figure it out for us, magically we can do it, i love it!