I hate to say it but I have had more than my share of patients fall after this type of surgery, normally with no significant repercussions. The screws and rods that stabilize this surgery are substantial and are very resistant to forces generated by falls.
Nonetheless, there are various problems that can occur after one of these falls. You could herniate a disc above or below the fusion, you could possibly displace a screw or you could stretch a nerve. If I had a patient with new complaints after a fall, the first thing to do is a thorough examination.
There are times that this examination can reveal a new and different nerve problem which would indicate a herniation above or below the surgery. Return of the same symptoms as the patient had prior to surgery might prompt me to order a new MRI. Certainly, new X-rays would be in order the moment this individual stepped into my office.