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You have set the only key you have to a MYKEY. And since you do not have the ADMINISTRATOR KEY anymore, you will need to have the key you have reprogramed to the vehicle which will clear out the MYKEY. It would be best to purchase a second key and store it in a safe place incase it happens again. Both keys will need to be programed to the vehicle and the dealership is the best place to have this done. It is a very common problem we run into at the dealership. We see it at least twice a week.
The keys are about $150. They usually charge 1 flat rate hour. At least we do at my dealership.
The administrator key is just a key programed to your vehicle that is not set to MYKEY.
Below is a section from the owners manual describing the MYKEY.