Finish off a back up?

after hours of backing up my files i had to turn the computer off (put to sleep) the system had not finished the back up ,i was on disc 17,when i tried to continue were i left off i got the message back up failed,when i tried to restart the back up it asked for disk 1 presumably to start the back up again why cant i just start the back up were i left off any help would be much appreciated don't fancy starting all over again with new discs