Most likely your bobbin thread has gotten too loose by the screw vibrating out a bit, or there may be some lint in the bobbin tension plate that prevents it from putting any tension on the thread at all. Make sure it's clean behind the bobbin tension plate (on the bobbin case) and then put another bobbin in. Test the tension by holding the bobbin and case by just the thread. You should have to shake the thread lightly to get the bobbin to unwind and drop down. If it just drops straight down as soon as you let go of it, there is not enough tension. Tighten up the tension screw a little at a time and keep testing, until it's tight enough that you have to shake the thread a bit to get the bobbin and case to drop down.