URGENT - accidentally touched a printers Drum Unit and now it's not printing well. Fixable?

It would be good to know just exactly what kinds of problems you're having with your print.

The drum cartridge in general is OK to touch, the only thing you need to be careful of to not touch is the actual drum tube/cylinder, which is usually a kind of blue color, that is at one end of the cartridge (it had a protective plastic or paper piece covering it that you should have had to remove before using it).

If you just barely touched it or brushed against it, it shouldn't really affect the print quality all that much. But if you have oily skin and left residue on the drum itself, like a smudge or fingerprint, then you can usually take a soft cloth or tissue and carefully, with increasing pressure if necessary, rub the residue off of the drum surface (I've done it many times for customers who've actually had labels get stuck on the drum surface and had to rub off the adhesive). If it is still there, then you can try a little alcohol on the cloth to see if that will help clean it (you've nothing to lose at this point, if you don't get it cleaned off you have to get a new cartridge anyhow).

But, if there's nothing on the drum surface to clean off, or the problem is widespread across the print that is more than where you touched the drum, then you may have another issue other than the drum that's causing your print quality problem. It could be the toner/developer cartridge, transfer roller, fuser, etc....

Mfc7840w Drum