Most of the times there is a a problem with your dvd reader. you might want to clean out your ps2 because of the dust that comes inside it it will not function 100%. If that does not work try to put your psII all togheter again. good luck.
Unplug Playstation, unscrew corner bottom screws. Take top off and locate a bronze bar. See if it's on track. If not, move it slightly until it makes a small click.