Could the wire have broken inside the rubber sheathing maybe,or come loose at the join to the actual keyboard?
I would just buy a new one as they are really quite cheap. Currys and all the big shops like them sell them for around �5.00
one way to prove is to try to get into the BIOS when the machine is starting up .... at that point the machine is talking directly to the hardware so if no joy at that point ... not much else is in the way
the best way however is - if you can .... borrow a keyboard and see if that works (a cord has two ends ... it's possible (though unlikely) that it's the mobo
if it's usb .... try all the sockets ... or try a mem key in the sockets to prove they are working