My office HL5150D printer toner light was on. I put in the toner cartridge from another HL5150D printer in the office but I still had the problem. When I put the original toner cartridge into the second printer, it then had an error of the amber toner light and a red status light. I switched the toner cartridges back but both printer now had the errors. It almost seemed like the "bad" toner cartridge may have affected the printer functionality.

SOURCE: Brother HL-5140, Status light is red and toner

It is very hard to find the reset instructions on the Brother Solutions site.

1. Turn the printer off.
2. Press and hold the Go button on the front of the printer, at the same time turn the printer power on.
3. When the Toner light is on by itself, release the Go button.
4. Push and hold the Go button again, release the Go button when the Paper light is on by itself.

SOURCE: red status light on. won't print,

Try this. 
http://welcome.solutions.brother.com/BSC/public/files/dlf/doc000071/HL5100_UG_ENG_3.pdf

SOURCE: Brother 5250DN Printer Toner light

Try resetting the toner counter manually;  * Turn off the printer and open the front cover * Press and hold the Go-button down * Turn on the machine while still holding button down, when all three LEDs are lit, release Go-button * Press the Go-button twice, and all three LEDs should be lit again. * Press the Go-button five times (six times if you have the High Yield toner) * Close the front cover and the toner counter should be resetted.

SOURCE: Toner light won't turn off

i just bought this printer last week, and printed out less than maybe 100 pages before I got the toner light. i ALMOST spent money on a new cartridge, but luckily i left my wallet at home. i came home, and checked the google, and beheld the solution to my problem in robdew's post. i also put a piece of tape (electrical) over the scanning hole on the right, shook up the cartridge a bit and i'm printing like a charm again.
if you're in my situation, don't listen to CW Oly; you'd be out $60+.

SOURCE: cannot remove toner cartridge from Brother HL 2170W

Had same problem. The instructions don't tell you that it is not just the cartridge that needs to be pulled out but the frame (black metal thing) inside which it sits. The whole thing pulls out. Of course, you have to press the green tab too.