Are the chain plate and upper turnbuckle bolt that is swagged to the cable in good working order?
If not you will need to remove the whole cable from the masthead to the chain plate and take to West Marine or other rigging supplier to be rebuilt. Look in The "Yellow Pages" under "Marine Equipment and Supplies". If they are good all you need are the turn buckle and lower bolt to order a replacement. You will get a new upper bolt with it but will not need it. Your lower and intermediate shrouds will hold the mast erect while you wait for parts.
If you are at all squeamish about partial repair or at all concerned about the condition of the upper parts of the cap stay then have the whole cable rebuilt. Make sure you check the other cables overall condition as well.
If you have comprehensive insurance you may want to have the whole rig checked by a marine surveyor. As a matter of fact, that would probably be the best bet. Then you could see who he would recommend for replacement parts.