Well even though she didn't adjust anything, maybe she still knocked something out of whack by cleaning and oiling. Have you pulled your bobbin case with bobbin out of the machine and tried to set the bobbin tension? Pull the thread out a bit and let the case/bobbin dangle. If it doesnt let out more thread, or isnt easily letting the thread out, its probably too tight. There is a little screw that you can turn to loosen or tighten the tension. The screw on the right is the one you want to turn.
If your bobbin just falls you need to tighten your tension.
Do you have a manuel? It might explain things better than I have. Hope it works out and gets you sewing again. Gina