Short answer. An eBike display that will not turn on is usually a display cable connection issue, a dead display unit, or a battery that is not delivering power to the controller. Reseat the display cable first, then confirm battery output.
Step by step
Reseat the display cable. Unplug the display cable at the display and at the controller. Inspect pins for corrosion, straighten any bent ones, and reseat firmly.
Try a spare display. If you or a mechanic have a compatible display, plug it in. Working spare confirms the original display is dead.
Confirm the battery is discharging. Turn the key on and check for any lights on the battery. No lights means the battery is off, not just the display.
Get a diagnostic if none of the above. Persistent no display with a good battery is a controller fault.