What to check? Oil level to start with? Is it the low oil warning or oil pressure warning?
If low oil light (yellow/amber):
could be that you ripped the sensor / cable off..
If pressure light (red):
Next, take the undertray off (if not ripped off) and see if there's a leak or see if the oil pan is scraped or bent. If it is and you don't have leaks it could well be oil pump pickup or if you done well the lot but that you hear very much.
Hope for you that it's only low oil or pan... At least that you can fix during a weekend and an old oil pan from scrappy.