The Canon Pixma MP780 is now quite an old printer. Over the years the print head slowly accumulates dried ink internally which sometimes can lead to the "Wrong Cartridge" message appearing on the menu screen.
This is often the cause however it can also indicate that your print head has died. Before you fly off and buy a new printer try removing the printhead and cleaning it to see if this is the cause. With Canon print heads they are very easy to remove and the dried ink easily removed under the hot water tap letting the ink flow into the sink.
If you clean the print head and the message persists then the print head has most likely died. The print head is by far the most delicate part of a printer and is often the part which dies first. Modern printers nowadays are a throw away item (as with most products) and to replace a print head is a waste of time as a new printer is not much more to purchase.
The best way to keep your Canon Print Head Clean is to use your printer regularly. When a printer sits around for long periods of time the ink left in the print head tends to harden. Remember a printer thats looked after and regularly used will serve you for many years (hopefully).