I have a Janome, too, and just recently it suddenly went out of whack, just as you described, with the top tension being...not right. Finally I realized it happened right after I had changed the top thread for just some quick stitching in color. I had used that thread before so didn't think it could be the problem, but I think somehow using that thread in conjunction with whatever was in the bobbin closed the show. Once I changed the top thread, all was well again.
We live and we learn...