Message 77. What does that mean? - Brother IntelliFAX 4750 Plain Paper Laser Fax

Machine Error 77 is a problem with the temperature of the fuser. The fuser is the hot rollers in the back that melt the powdered toner into the paper, making your image permanent. It's temperature is regulated with a few feedback and safety features to shut it down in case the regulator goes haywire or the sensors stop working. The redundancy in the system is because a runaway heater could be a fire hazard. If any one of the sensors involved doesn't see the temperature it expects, it shuts the whole works down, and puts up an error code.
If this started when you pulled a paper jam out of the back of the machine, you may have accidentally unplugged one of the fuser's cables, but it's unlikely. If you did, it should be hanging in plain sight in the back of the machine.
More likely is that the fuser's heat lamp has burned out, or the thermistor that senses it has gone bad. Some machines have a fuser that can easily be pulled out and replaced, but this one is more difficult, and the teflon-coated rollers are easy to scratch or gouge. I'd recommend taking it to a Brother-authorized service center. Call first and ask if it's worth it.