Did you rip the cable when you took of the mid plane?
You will have to solder on the cable. If there is some solder left on the spots you may be able to use them. Make sure all the yellow tape is off. Use a very, very fine tip on your soldering iron and high temperature. Put the cable in place and hold it in place with tape so you see the soldering spots through the holes in the cable. Apply the iron very quickly to each spot.
I am not saying this is the best way. The best would be to clean all the solder from the contacts, tape the cable in place and apply new solder to each spot. Still using a small tip and high temperature.
This is not as difficult as it may seem. I know it seems scary but I fixed it several times and I am a novice solderer. The important part is to get rid of the isolation over the spots and to use a fine tip so that you can use high temperature on the iron. If you are afraid to do it (I don't blame you!) someone with simple soldering skills should be able to do it.