There's not enough information here to begin to diagnose this. My first thought is that this is a signal issue. That could be caused by an antenna issue, or a sensitivity setting in the headunit. If it isn't one or the other, its a hardware problem in the headunit. That usually means sending it to a technician.
If you've got a manual, look at the FM tuner section. If don't have a manual, download one from Pioneer.
First question I have is what station frequency does it keep returning to? Is it 87.9 or 107.9? (assuming a US unit). That might indicate that the tuner could not find a frequency which it could lock onto. Those are the end points of this tuner's FM frequencies.
On the FM screen there is a signal strength indicator for the tuned station. Of course, if it is tuned to a non-existing station, there will be a poor (or none) signal strength. Unfortunately the manual doesn't show how to interpret this indicator, but the screen-shot does show what it should look like with a good signal.
On the bottom-left corner of the FM screen, touch the gears icon (Settings), then touch [Local] then touch [On]. Make note of the sensitivity level (1-4), then set it to 1. The higher the number the more filtering the tuner does (eliminating weak stations). Level 4 eliminates all but very strong stations.
Exit out of settings and try tuning to a station manually. Does it work better? If no change (or even only a little better), go back into the settings/Local and turn if Off. The try tuning again. If no (or only slight) improvement, I'd next look at your antenna. You may need to remove the headunit to inspect the wiring.
It is possible the antenna connection is loose. If your antenna is in the window glass it is amplified and might be powered by the headunit (depending on how it was initially installed). The headunit's power wire (blue wire in the power harness) might of become disconnected. It is possible the installer used some other fused switched power source, So you'd need to check your wiring. It is also possible that the antenna's ground wire is loose. Or maybe the antenna is faulty. I don't know how to diagnose this. A stereo shop might have instruments to test this.
If none of the above fixes or significantly changes the symptoms, it is likely that your hardware tuner is malfunctioning. If you are technical enough to service the unit yourself, you likely wouldn't be here asking for help. So I'd recommend sending it for repair.