All DVD transports, like any moving motor, eventually lock up or seize due to lack of lubrication. With a DVD motor, this is problematic, as the electronics inside the unit expect the disc to ramp up within an allotted amount of time to read the contents. But it is a problem you can solve.
Remove the top of the unit, using the appropriate-sized Philips screwdriver. Consult the manufacturer's warranty details to determine if this will void it.
Apply power to the unit, and eject the tray. Lay two paper towels over the internals of the unit, with the paper resting on the sides of the chassis. Power will remain on during this procedure.
Spray three short bursts of WD-40 (or similar lubricant) under the plastic disc that grabs the DVD and pulls it into the motor and driveshaft (it will be a quarter-sized black or grey disc that is attached to the spindle motor). Let settle for a few seconds.
Place a commercial DVD into the tray, and allow it to spin up. If it does not, your player may require professional service.
Gently clean up excess lubricant sling (throw off that occurs when the motor spins up) with paper towels and cotton swabs, paying special attention to the laser assembly.
Re-attach the top of the unit.
Avoid touching the internals of the player with power applied. Remember to carefully clean the laser lens with a cotton swab immediately after this process.