If it is unbonded, there is a connection typically referred to as a "dogbone" type of connection. These are used alot in parking garage type repairs. You could use 2 of these (one on each end) and splice in a new piece of strand and restress it. These however, do take up a lot of room, so cover could be an issue for you.
If bonded, check the development of the strand. After a certain point, the tendon should regain its full capacity since it is bonded to the concrete. Do you need it for strength or just crack control? Check with VSL. They are good about helping with different type of repairs. Not sure what you would do in your case, but I would think you would need to use some sort of external anchor and a post tensioning rod. You might be able to chip away some of the concrete and embed this in the slab, and "splice" across the damaged areas, but this could be tricky. Again, check with VSL or another PT supplier.