Um...black will only print whatever black is selected. A good nozzle check should go (P/M)K, LK, LLK, C, M, LC, LM, Y. If both PK and MK are never printing, there maybe air in both lines. I find it surprising that PK would have worked at all without priming. It could be that it printed for a while and now that bubble of air has finally reached the head.
You can prime the cartridges using a regular syringe by taking the clear plug out of the cart, on the side with the blue label. There's an arrow pointing towards it. Insert the syringe without a needle and pull until ink comes out.
Regardless of what method you use to prime, MAKE SURE THE ORANGE CAP IS REMOVED from the air hole!
Your maintenance cartridge will fill quickly when you do Power Cleanings. It is suggested to have the resetter and a spare tank on hand at all times.
The connection error should be unrelated to the other problems you are having. Turn the printer and computer off, unplug the printer cables, plug in printer power, plug in USB into the USB port on the printer (careful: the USB cable can fit into the LAN port. I've done this before, and obviously it doesn't work ), then into the computer. Power everything back up. If that fails, try reinstalling the printer software on the computer.