I have a Zenith 2 light (halogen) flood light that worked fine until a couple of months ago when we needed it most! There was a guy in our driveway in the middle of the night casing our cars. Our cameras picked him up but the flood lights did not and have since not worked. When I turn them on they stay lit (20 minutes in this case) as set. When I approach the sensor at night the little red light flashes as if it's picking up motion but the lights do not turn on. Should I just replace the unit or is there another fix? I have the same set-up on the back of our house and it's worked fine for many years. I have the setting set the same as the light on the back of the house. Only difference is that the problem lights are switched on every evening as they are wired with other outside lighting
Replace the Fixture
In this particular case, since the red light flashes, it sounds like the sensor is OK but it isn't activating the relay for the light. But on an older halogen unit it is not worth repairing - just replace it with a new LED fixture. The electronics do wear out over time, especially with outdoor fixtures, and you will save money on electricity with a new LED fixture.
Look at the existing bulbs to find the power (watts) used and then translate that into an equivalent LED. For example, if your existing fixture has 2 x 100W Halogen bulbs, that is roughly 2,000 to 2,500 lumens according to this chart. Replace it with a new fixture like this one picked at random from Home Depot and you get 2,400 lumens for 26W. At 0.13/kWh (average US according to a quick search), saving you $8.25/year for each hour/day that the light is on (motion detected -> lights actually on; the motion detector (old or new) uses very little power).
Generally on motion sensors of that era, the motion sensor device and the lights are separate pieces of equipment. The motion sensor can be replaced separately.
That would still leave you with a clunky old incandescent motion sensor, but they aren't on very often. Besides, nothing keeps you from changing the bulbs to LED. Make sure to select a motion sensor module that's LED friendly.