Cannot access BIOS, keyboard won't work...

I am the classic friend that fixes all problems concerning computers.
One friend gave me his laptop computer an HP compaq 6510b with some monitor issues (later i discovered that the inverter and the lamp were broken) to reach the main lcd connector i had to disassemble the power button panel then also the keyboard. Now i have reassembled it but i can't type anything, the keyboard seems to be dead, after magnifying analysis nothing seems to be broken on the ribbon cable, it is still entirely perfect.

Notes:
1-Before my disassembling the keyboard was working
2-i have powered once the computer without connecting the keyboard
3-the laptop is totally messed up with both Vista and Windows 7 installed on it
4-I have tried to remove the CMOS battery and wait for 20 minutes, nothing.

It is driving me crazy, i don't want to buy another keyboard, but until it will be absolutely necessary i want to know ALL things i can do to save it.
PLEASE HELP ME!!!!
(sorry for the bad english...)