If this has just happened, and you have printed from Photoshop before, then check your printer. I had a similar problem, but the printer status said there was about a third of a cartridge left, not that then... there wasn't; it was telling me porkies! As for CMYK, if it's not going to a printers(just on your home inkjet) then RGB is your best bet. One case where you would use CYMK on an inkjet printer would be with a PS(postscript) printer, if you intend to do proofing - that is ckecking the results of a press printing.