I have an Office Jet ProL7780 All-in-One. It says it has a carterafge jam and to open print cartrige doort and clear jam. I have checked in every door, taken out every tray, and no jam exists. Yet after pressing OK, it shows trhe same informatyion. I can not get it to cvlear even by shuttring down and rebuting.

Open the printer so the catridges allign so you can take them out like you were replacing them. Once it's in a position that you can take them out, unplug the power from the unit. Take the cartridges out, close the unit and plug the unit back in. Open it up, put the cartridges back in after they align to allow it. Close the unit.

If that doesn't work, uninstall the printer and reinstall the printer drivers again.

Let me make sure I am understanding your problem and getting all the information. You have a cartridge jam. Have you tried removing and reinserting the cartridges? How long have you owned this printer and how long have you had this problem?
Thank you,
Lee