PS2 Slim won't read dvd games
Blocking the sensors did you in I'm afraid. What has happened is the *DVD* read mechanism has gotten goofed because of this, but the *CD* read mechanism is still ok. Maximo and a few other PS2 games were actually on CD, not DVD. If you look on the back cover, you will see the Compact Disc logo (in the lower right area on the box for Maximo). These games have a blue bottom disc instead of the silver bottom.
You can attempt to re-align the laser which is a fairly lengthy process and there are numerous guides out there to do this. Here is one: http://arstechnica.com/articles/paedia/hardware/ps2.ars
Note that this guide is for the larger model but the concept of lens adjustment is still the same and a quick google search will provide more in-depth detail. Since you have already opened your system, this may be an option for you. If the PS2 was purchased new as you said, it might not be the worst idea to exchange it for a new model as the lens going out this soon might be an indication that the unit wouldn't have lasted that long to begin with.
Also just a friendly reminder that opening your PS2 voids your warranty and double check the return policies of where you bought the system.