[Mart]

Take the top off and have a look inside. If it's killing the motor it would tell you it is in two gears at once.

You will have to investigate. You will need the correct top to match the gears.

You may have fitted the correct top but not engaged the forks in the gears correctly.

take the top off and manually move the gears to the neutral position. See if that frees up the motor (watch for splashing oil).

Mart.