Welcome, katierose.
Did it work as expected
before you adjusted the tension?
Did you mark where it was before changing it?
Do you have a manual for your machine? If you don't you can download one from
http://www.singerco.com/support/instruction-manuals
Near the end of the manual there should be instructions on "Remove And Replace Bobbin Case" before instructions on lubricating the machine. I would suggest making sure that area is clean and replaced correctly. Perhaps the bobbin case holder is not seated correctly. Maybe it got bumped when you were trying to adjust the tension.
Janey - Neat people never make the exciting discoveries I do.
ETA - I'm sure it probably won't help this particular problem, but there is a thread here --
She's real fun, my 401... about using 30W oil to help this series of machines (
only where the green arrow is). See posts #9 & # 15. It also has regular oiling places with regular sewing machine oil.