I'm guessing you have a faulty throttle control, and it is sending too high a voltage to the controller, when you turn the power on it is causing the power safety cut-off feature to activate.
If you have a spare throttle control
(twist or thumb), plug it in an see if it works correctly, if it does, then it would indicate that the original throttle control is indeed faulty.
If you don't have a spare throttle unit to try, you will need to try the following:
With the power turned on, unplug the throttle and you should hear three beeps. If you then plug it back in
with the wheel off the ground, you will probably find that the motor will start to spin at the same speed that it was stuck at before.
You will need to check the voltages at the throttle connection to determine whether it is the throttle control itself, or a poor ground connection to it.
If you connect the black meter connection to the negative Battery connection
(-Ve) and check you have a supply voltage of ~5V on the Red throttle connection.
Now see if there is any voltage reading on the Black throttle ground connection.
(It should read ~0V if the ground connection through the harness is good)With the throttle released you should have a signal voltage of ~0.8V on the White
(or possibly Green) throttle connection.
(With the throttle fully open you should have a signal voltage of ~3.6V.)If the ground reading is ~0V, and the throttle signal voltage is greater than 1.25V with the throttle released, then the throttle control is faulty, and this will cause the problem you've described.
From your description, I reckon your throttle control has failed with the signal voltage stuck at around 2.5V.
With the throttle unplugged, you should be able to use the bike at half throttle by holding in the cruise button.
Let us know what you find.
Alan 
