I lost the game disk!!!! No!!!!!!!!?

Noooo! I lost my GTA: SA disk! I don't have the game installed on my computer but when I wanted to install it again, when I opened the case, there is NO DISK!!!!! I checked everywhere but nothing! Could it be my mum's friend's son who took/borrow or even stole it when he came over for like a month ago? I don't know because I think I remember playing it the days after he left but I don't have it on my computer because I wanted to reformat it and I can't play! How do I play it otherthan buying a new one?