Flashing "wrong cartridge" and won't print.

Not normally but the black print head nozzles maybe blocked.
Inkjet printers need to be used frequently to prevent ink drying out and blocking the print head nozzles.
If the print head cleaning maintenance routine does not clear the blockages, then print head needs replacing, but it may be expensive.
It may be more cost effective to buy a later model and better printer than replace the print head.

You will need to reset the printer

Here is the Cannon LINK for supplies.

If you need more help come back.Post / thumbs / testimonial are welcome!

IS THERE A FUSE OR RESET BREAKER ON THE CANON PIXMA MP780 PRINTER

Check cable. Sounds obvious but bad connections are a problem.

Check Cartridge, rebuild?

Is the print stopping or is it fading away?
Fading away a cartridge issue.

Stop printing at the same point. This may be a corrupt driver issue, reload driver. Remember to remove the printer first then reload.

ink tank

On this machine it means the printhead is bad. QY6-0049-000 around $40.00

Try to reset to factory .
Remove ink cartridges and print head.
Power off machine and unplug for 10-15 minutes.
Restart machine and install head and cartridges.

Reboot ur printer.

PIXMA MP780 "WRONG CARTRIDGE" MESSAGE?
Here is what I did and it now works just fine.I turned off the printer and unplugged the power cord, and unplugged it from my computer. I then removed all the ink cartriges and removed the print Head I got some rubbing alcohol put it on some toilet paper and wiped down the back and bottom of the print head.Plugged everything back in and It now works better then ever.