The display shows 0 or the wrong speed. Most likely cause: a speed magnet that has moved or fallen off, a damaged sensor cable, or a wrong wheel-size setting. Try this first: turn it off and on again.
The spoke magnet must pass close to the sensor on every turn, or the speed reads zero. Photo: AndrewDressel, CC BY-SA 3.0, via Wikimedia Commons (resized)
The display works out your speed from a sensor, either inside a hub motor or a separate sensor and spoke magnet, plus the wheel size you've set. If it shows 0, the signal isn't arriving. If it shows the wrong speed, a setting is usually wrong. On many bikes, a missing speed signal also cuts the motor's help.
Step-by-step checklist
Work from the top. Steps go from free and easy to shop jobs. Switch the bike off before touching any connector. Tick each step as you go. Your progress is saved on this device.
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When it's time for a shop: if you've worked through every step and the problem is still there, take the bike to an e-bike shop. Bring your notes on when it happens, plus the code.