Since you are talking Singer 201, I strongly advise you not to take the bobbin assembly ring off. It's the top ring on the area that holds the bobbin. It's a real bear to get back on. On the other hand, if there is dirt or thread in there and you can't get it out any other way...
Vacuum, do not blow air into it as that will drive dirt deeper.
Try those things first. There are other things to do if those don't work.