Double check your top tension setting. Sometimes this gets moved without notice. Also, when you rethread the top, Make sure the pressure foot is up when you place the thread through the tension plates. Hold the thread at the top and the bottom and make sure it has popped into the disk. Then lower the foot to thread the needle.
For the bobbin, make sure you have the thread facing in the right direction. I have accidently reversed the bobbin and it caused a lot of frustration.