My PS3 has a black screen, and discs don't show on XMB.

I recently cleaned all the dust from the inside of my PS3 (disassembled only to the point where i removed the power unit and blu-ray drive), and put it all back together, being very careful to follow guides on-line. When i start my ps3 now, it comes on fine. If I select any installed games, PS Store, all I get is a flash and then a black screen. When I insert discs (games, dvd's or blu-rays), they don't get recognised or show up in the XMB.

I've rebuilt the filesystem, checked every connection that i touched during the disassembly, and everything is as it should be.

Can anyone tell me what is wrong?