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.