Happens to me always and again. There are days when I am in good form for learning something, on others I am not. This is something I have learned to accept. I try to play harmonica and tenor banjo mainly (there also is a mandolin, strumstick and whistles), sometimes picking up a different instrument helps to get over those blockings. I noticed that fiddlers are often one-instrument-people, but just try something different - I would suggest the harmonica, since it requires no intricate fingerwork like a stringed instrument does.
Best wishes
Ernest