There is a black line down the middle of the page when we copy.

Is the line from making a copy or are you printing ? Since you changed the cartridge then the line may be from the fusing roller.

The problem may be from either from the toner cartridge or the settings of the printer. change the toner first and see whether he problem still persist and also make sure that the settings of the printer on your system is ok, if the problem still persist after doing all this then it implies that you need to service your printer.

The fuser is located in the back of the printer. I think it's held in by to screws. It's about hte same size as the toner cartridge. Please visit these websites.

This is the fuser kit.
http://store.printerprinciple.com/products/rm1-3044

The service repair manual.
http://www.scribd.com/doc/15253671/HP-Laserjet-3055-Repair-Manual

Fuser Replacement is on page 170.

You may need to replace the toner cartridge or the fuser assembly.

If you open the toner cartridge door and see a black line on the roller, you will need to replace the cartridge.

Does this happen both when copying and printing?
For printing solution, try another toner cartridge.
If lines continue to appear when copying (primarily through the automatic feeder) then lift the auto feeder, and clean the glass -mainly the narrow piece on the left side.

please rate solution :)

Try wiping the light bar where the page goes in with your finger.

open the top cover on the left hand side glass plateform with a narrow piece glass clean it with alcohol.

clean the print heads and run clean cartridge utility from printer properties window.after that
disconnect the power and usb cable from the back of the printer and press and hold down the
power button for 30 sec and then reconnect the cables and try again...open the backdoor and
try to move the rollers in upward direction.

I will give you all the reasons I can think of.
1, If the line is only there when running through a document feeder and not there off the glass. The small strip of glass directly under the document feeder is dirty.
2. The drum has a scratch on it, or the cleaning blade is not cleaning the drum properly. Would require that cartridge replacement.
3. The fuser roller may be work in the middle. You would notice some reprinting , or ghost copying if that were bad.
4. Less likely would be some piece of fuzz in the light path, or a small piece of paper or tape in the paper path smearing the ink across the page.
If your machine prints, see if the line is on a print, if its on a print and not on the copy then it is scanner related.