Hello. My name is***** can help you with your question.
With the front door open and looking at the front of the unit when it's plugged in, do you see a button or a blinking light? You should see a button on the front and holding that button in for 10 seconds should reset the drum. If there is a light blinking, then pressing and holding the button should cause the blinking light to become solid. When it does, that means the drum is reset. Let me know if that works for you or if you need additional assistance.
Also, I should mention that if you used a third-party or remanufactured toner or drum, that can cause a problem as well. If that's the case then as much as it pains me to say it, I would get an original OEM brother version of each.