I'm no expert but have followed many threads on the SP-540, and I know that doing any thing like what you did, you first have to UNPLUG the machine and and make sure you have discharged any static from your body before touching any sensitive components near the print head. Because you can blow a fuse on the head board if you are not careful.
Another thing, if you unplug the ribbon cable, make sure you installed it correctly, check to see if any of the leads are bent and not making contact.
You may need to check the fuse, I have not done this, but know if you don't have continuity across the fuse, then you blew it. The fuse is a very small surface mount component, however it is something that many have replaced them selves (if you know how to solder). One other tip I have read about, don't use the same small fuse going back, but replace it with a fuse holder and that way you can change fuses easier in the future.
Good luck