Your lens on the reader has gone out of alignment. There are several places to be found with a Google search for "realign PS2 lens" that could help remedy the situation.
You will have to take the top off the PS2 in order to follow their instructions- personally, unless you are very sure of what you are doing I would not recommend this. We've seen several people that have taken the top off for whatever reason and somehow knock a cable loose, rendering the system completely dead. I would prefer you find someone that has some experience in these sorts of things and have them look at it.