For dsPIC30F/33F and PIC24F/H devices, a reserved bit in the peripheral control register (usually either bit 14 or 5) is used as a Freeze bit by the emulator. If you have performed a write to the entire register, you may have overwritten this bit (the bit is user-accessible in Debug mode). To avoid this problem, write only to the bits you wish to change for your application (BTS, BTC) instead of to the entire register (MOV).