APEX Digital VCR&DVD PLAYER ADV-3800 will not play DVD's.

Actually the Player indicates on the counter that the DVD is playing but I have no on-screen display, not even the DVD Player 'blue screen'. It will play VCR's but ever since I plugged in the Comcast Cable 'On Demand' box I could no longer see or hear DVD's playing. I tried what the user's manual suggested: turn off the player and replug it back in ... I also tried several different DVD's with no change.
I disconnected the Comcast Cable video and audio cords from the Cable Box and TV and placed video and audio cords in the OUT plugins in the back of the APEX Player and into the IN plugins in the back of the TV. Having done that, sometimes the APEX Player plays with no sound or display OR sometimes when I place a DVD in the player; after getting the ''Load'' message on the front of the Player, I get the message ''No Disc''. I've torn everything apart and put it back together but the APEX Player does not respond properly. OR is it the Cable or the plugin cords or my TV or what? I must say, the APEX Users Manual is very hard to discipher. Please help!!!!!!!