Failure of anterior cruciate ligament reconstruction is thankfully uncommon however the rate of failure is somewhere between 3 and 10% in the orthopaedic literature Grafts can fail due to either technical problems with the first operation or due to a further injury to the same knee. A reconstructed ACL is very strong once it has fully healed but can be injured during the rehabilitation period and this is why return to contact sports is not recommended for 5-6 months post surgery.
Following the rehabilitation you are as likely to tear your opposite native cruciate as you are the graft but a big enough injury can cause graft rupture.
If your knee becomes unstable and you cannot return to sports/activities a revision (redo) reconstruction may be needed.