I let my friend use my computer, and she wanted to play cookie clicker after I showed her. I left to go to the bathroom, and when I came back, she had hacked the game so it said infinity in the place where the number of cookies usually goes. Please help!
You can either:
Fully wipe your save, achieved by going to Options > Wipe save, shown below:
Or, assuming the only thing they changed was the amount of cookies you had, press F12 (Chrome) or Ctrl+Shift+K (Firefox) to enter the console, and type
Game.cookies = 0 to set your game's cookies to 0. You can change that number to whatever you want if you want to reverse the changes.
Keep in mind that modifying
Game.cookies will grant you the Shadow Achievement "Cheated cookies taste awful", so if you don't want that, you'll have to wipe your save.