Hotpoint RVM1425

Question

I heated a muscle relief heat pack in the microwave. I think it's filled with sand?? After that the microwave wouldn't heat anything. I still have power and the glass plate still rotates. When I try and select a power level I get the message..power level may not be changed at this time. Also, the small fan that automatically activates when using tha microwave sounds different..a deeper roar/sound?? Any help would be greatly appreciated!! Thanks, Mark