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If the transmission is staying in Park (won't roll, wheels are locked) then you may have a shift cable that has broken or become disconnected from teh transmission, or an internal failure of the linkage between the shift lever and the valve body. have a friend move the gearshift lever, while you watch the lever on the transmission to see if it is moving. If it is not, then the cable is disconnected or damaged.
If the vehicle is coming out of Park (car can be pushed, wheels not locked) and teh fluid is full, then you have an internal transmission failure, such as a failed pump and diagnostic pressure testing will be neeeded to begin to assess the failed components.
If the lever on the transmission is moving, but the parking pawl (lock) is not releasing, then there are 2 things you can do:
(1) Make sure that the nut retaining the lever to the shaft passing into the transmission case has not come loose, allowing the lever to rotate on the shaft.
(2) If the shaft passing into the case is definitely rotating when the lever is moved, then you will need to drop the transmission pan and look inside the case to see what part of teh shift mechanism has either broken or become disconnected. This often isnt as scary as it sounds; most of the components involved in this mechanism are pretty visible once the pan is removed. If you are unsure waht you are doing at this point, I would suggest having it towed to whatever repair facility normally does your service work; it most likely is not a terribly complicated repair; you likely need some sort of retainer clip or linkage component, not a new transmission.