cd player not reading a disc? Laser dirty?

What causes this erratic situ where sometimes the player won't fire up a disc that it was playing a 1/2 hour before?
Could it be the laser is dirty? I clean the disc(again) to no effect.
That's a tough one. If you're cleaning the disc properly, then that eliminates this cause. How old is your player and did you ever consider purchasing one of those CD player laser disc cleaners? It's nothing more than a CD with a tiny micro brush that is supposed to clean the laser lens as it passes by during play. I know these are controversial to audiophiles and enthusiast with mixed views on this. I have one and use it sparingly. Have you tried one? What about the drawer? They can become a bit dusty too transferring dust to the disc.
This describes any and all variables, of course, always start with the simple first.