Hi,
What is the model number of your DVD player?
You can try power resetting the unit and check if it helps. To power reset, turn OFF the DVD player and unplug the power cord from the electrical outlet for 30 seconds. Plug the power cord back into the electrical outlet and turn the player ON. Attempt to open the disc tray.
If this doesn't work and since you do not state the model number of your DVD player, try the following:
Simultaneously press the POWER + STOP + EJECT buttons. If the disc tray still does not open, simultaneously press the DISC1 + STOP + EJECT buttons.
NOTE: This step may not apply to all models as DISC1 is not available on all DVD players.
If this does not work and if the DVD player has DISPLAY and PREV buttons, press and hold the DISPLAY+PREV buttons for two seconds to unlock the drawer.