LoL is "damaged and should be moved to the trash"

Oh boy, here I go. I've had LoL for about a month or two, and this morning it stopped working. By that, I mean that it just continuously bounced up and down in my dock/sidebar (implying that the app is starting up). Though it never opens. Ever. So I uninstalled it and reinstalled it (I made sure to do it properly), but now it says that it's a damaged file and can't be opened??? I'm getting really frustrated and have been working at this for over two hours with no solutions. Please help.