singleplayer

yeah, hi. i downloaded minecraft 1.7.2 on thanksgiving, and it worked perfectly. but yesturday, my grandpa deleted it. i redownloaded it again today, and now it's not working. whenever i click singleplayer, like 3 or 5 seconds later the screen will say "Minecraft 1.7.2 (not responding)" but multiplayer is working just fine. what do i do?!?! i'm so confused! one day it's perfectly fine, then the next fart noise not responding!