How to repair after drilling through an electrical cable
Now what do I do?


I would suggest shutting off power to the building, double checking that the damaged cable is not live then tracing it to see where it goes.

It probably means cutting out the damaged portion and splicing in a section of new cable using joins that are approved by your local code for use in inaccessible places (that is, not in a junction box with a cover plate on a wall surface). 

If your local code doesn't allow this you'd probably have to remove all the damaged cable between accessible junction boxes.

See Can cable splice devices be concealed?