Fortunately there is more than one tendon available for grafting. And if you don't use your own tissue, it may be possible to obtain tendon grafts from a donor bank.
In the case of an ACL repair, the surgeon will inspect the damage first in order to decide what has happened and what to do next. The previous graft will be examined for partial or full-thickness tear. In some cases a revision or repair of the graft is possible.
If that's not the case then the knee is reconstructed again. If your surgeon used the hamstring tendon last time, then the patellar tendon is the next logical choice.
Scientists are also looking at the use of the very strong quadriceps tendon. They found that even after taking a piece of the quadriceps tendon for grafting, the quadriceps tendon is still stronger than the patellar tendon was before a piece was removed to use as a graft.