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Insert more carefully. Observe all lights and indicators - is anything different from the usual?
If not, then you may simply have a defective magnetic safety key. You could try replacing it.
is a source for a replacement.
If the key has been dropped, struck, run over by a vehicle, or similar rough treatment, then even if the magnetic part is still attached, it may have been demagnetized by shock, and replacing it is more likely to be needed.
Then there's the possibility that your treadmill has a defect, a defective component that is disabling its operation. In order to tell what might be the problem, the above careful observation is needed... and let me know what you have observed, and what you think.