My son put the battery terminals on wrong ,positive to negative and then I changed them back but can't get any power in the car now. What can it be?

 

dam that was genius most likely popped the main battery fuse. Check all the main fuses in the engine bay fuse pannel and then check the fuses in the passenger compartment. also check that the battery itself wasn't fried.your battery fuse will look similar to this.

 

You would think the sparks that were flying would be a good indication.