If you turn the key on and both filaments are lit, then you have a stuck, maladjusted or shorted brakelight switch. Rear one is located on the inside of the right footpeg mount. Front one is a small switch on the front brake lever assembly. Unplug each switch to find the bad one. Repair or replace as necessary.
If you just get the dim filament with the key on and brakes applied, the bright filament is likely broken. A fairly common problem on some Monsters. Get another bulb at your local auto parts store. If you put that in, and it still doesn't light up when you step on the brake pedal or pull the brake lever, try disconnecting and then reconnecting the electrical plug on the taillight assembly. It's kind of unlikely that both switches failed.
Your owner's manual has a wiring diagram.