All good advice above. Times when your fingers refuse to do some simple task of co-ordination should be welcomed as real learning opportunities. Practice slowly. If it doesn't work, halve the speed. Go on halving the speed, however ridiculous it may seem, until you can do it.
Another useful idea is to break something difficult into smaller pieces (NO, not the violin!!!). Sometimes the transition between just one pair of consecutive notes is the real problem. If you can identify them then you can isolate the problem and deal with it intensiveley. It may not be fun for a day or two but the end result will be that all your playing will get better.
Oh yes, and it does come and go, and after a session of intensive practice it may not seem better... but come back two days later and you may be surprised.