check the connections into the display - there are two cables going in to it, one from the mode switch, and one from the motor - they are push fit, if they are loose it won't turn on.
If they are ok then pop off the side cover on the motor, left hand side, and check the connection there are pushed in correctly.
If this happens to you just remove the top bottle cage bolt and with the battery in the bike use a small allen wrench like 2.5mm and turn on the battery. The on-off button has a small battery itself and if the bike sits for a few months it losses its charge, once the bike is ridden it will charge back up.
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