Probably the disk has got caught on the lip of the spindle - sometimes happens when the PS2 is vertical without a stand but can happen anyway esp. on earlier models.

Look up the "Shit!!!!!!" thread and one of my posts detailes which screws to undo to get the top off (at least on my PS2) andpoints to the 4 smaller screws to get the DVD cover off (carefully). Then ease the disk over the spindle lip to get it free and lift it off. Put it all back togther and it shouln;t happen again too soon.

Of course that may not be the problem, but if it is you have the answer.