Canon iP4500 Print Head Problem

Hi,

I'm using a Windows 7 PC and a Canon iP4500 printer. I printed something a couple of days ago which worked fine and I haven't changed anything but I just tried to print something and received this error:

"A print head is not installed or the installed print head is not compatible with this printer. Install an appropriate print head and close the cover of the printer."

I tried cleaning the contacts on the print head with isopropyl alcohol, but it didn't help.

Please can someone suggest how I can get it working again?

Thanks,
Joe