What happens if I abuse meth while I'm taking naltrexone?

Nothing good. By "meth, " I'm assuming you mean methamphetamine. Nothing good happens when you abuse this substance. There are some ongoing studies of Naltrexone to decrease methamphetamine use. These have some positive results -- but not 100%. Taking Naltrexone while abusing meth will not change the damaging effects of meth. Please, get help -- hopefully from a specialist in addiction medicine.
Probably nothing. Regarding the use of the two drugs, one legal and one illegal, there is no reported drug to drug interaction. That does not mean you should combine the two. Certainly one should not be using methamphetamine. Seek help and counselling from your pcp, and addictions specialist or a mental health provider. Just don't mix the meds, regardless.