yesterday i tried to update my bios whilst i was updating my son switched off the pc accidently , then i tried to restart my pc but it would not boot at all . can you please tell me how i can reflash my bios to how can i get my pc to boot up again?

Unfortunately this is often a fatal condition.

What is the make and model of the computer and or motherboard?

Perhaps their web site might have some hope. In general the BIOS chip would need to be repaired. The specifics would depend on the motherboard in question.

Just for example this is from the manufacturer EQS:

you will need to take your system apart, remove the Flash BIOS BIOS IC from the socket, look for some experienced user or computer shop that have some form of EPROM writer device, and re-write the Flash IC chip's proper content. Similar process also hold for power failure in the middle of a BIOS upgrade session.

http://www.eqscomputers.com/products/mobo/