Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
Is the black line in the same place, for every page produced by the printer ?
Is it continuous along the whole length of the page regardless of the matter printed upon it?
And, what consumables or supplies have been replaced in the printer so far, recently?
If the first two questions' answer is 'Yes', then the 'drum' might be bad, but the toner cartridge is possibly a source for the problem, and also the fuser roller(s) might have a scratch on them - which would cause a black line on all printouts too, but might be different along its length, or periodically interrupted along its length.
So I advise that you examine the fuser rollers for a scratch, possibly filled with caked-on toner, and then proceed to remove the toner cartridge, give it a gentle shake to and fro several times (but not over your clothing nor furniture, in case it leaks.) Then put the toner cartridge back into the printer, close it up, and see if the black line has changed any in the next two printouts.
(Also I must ask: is the black line appearing on prints from the computer? black lines on scans and on photocopies are different in origin from black lines on printouts.)