When i play gears of war 2 on my xbox 360 it says this disc is unreadable? help fast plz...?

hi, i am in chapter 4 act 4 in gears of war 2 now about 10 minutes ago it suddenly stopped and said this disc is unreadable.. this is the first time i get this message on my xbox.. i bought the pro xbox 360 and unluckily the pro is by 400$ plus cds very expensive.. so what shall i do. plz don't say change it to ps3..