aliasing to a 50 Hz sine wave sampled at 5000 Hz sampling rate?
You do have zeroth frequency in your signal indeed. If you don't, your signal would be centered around zero volts. Zeroth frequency is a DC component.

If you want to measure the frequency of an AC component, center the signal around zero volts (subtract the mean). By the way, FFT is not a good instrument to measure frequency of an arbitrary signal because it uses a fixed set of harmonics, which may or may not coincide with your signal (and FFT will give you a maximum on a nearest one).