Get it all nice and clean and oiled. Rewind the bobbbin and rethread the machine and put in a new needle. If all of this does not work; gradually up the needle tension. I never touch the bobbin tension because it is a big pain to fix. If you have used a speciality thread in your bobbin case you may have to adjust it. I have a "spare" bobbin case for speciality threads. I only use regualr sewing thread in my main bobbin case. I painted a dot of red finger nail polish on the "spare" case. If all the above fails you may have to take the machine in to have the tension fixed. Sometimes the disks or springs need to be replaced. Get a quote before letting them fix it.
Connie