I have a stanley garage door opener model 1200.51 which is chain and cable driven. The problem i am having is each cable came off and i want to know how to reattach them.

The most important thing is to understand that the door spring can KILL YOU. If you are not absolutely certain that you know what you are doing, call a garage door guy. Many poeple get SERIOUSLY hurt.

So, you need two perfectly fitting rods to fit into the spring tightening end of the spring..
Release all spring tension. Release the opener.
Level the door with shims if necessary.
Install the first cable and turn the spring main rod to hold tension.
Put a vice grip on the main rod to keep it from unwinding any more
Release the 2nd cable spool and thread the other side cable. Turn the spool to have the same tension as the other cable.
Then wind the spring about 6 revolutions. (35 flats or tool insertions).

Verfiy the cable is still threaded correctly on both sides.

Correct spring tesnion is when the door will stay at 1/2 closed.
REconnect the opener and good to go.