So a few days ago my LoL decided to randomly stop working. When I try to play, it gets all the way to where I can click "Play".
Here is what I do: Click my shortcut -> LoL symbol pops up in the middle of my screen, with the error box "Another instance of the launcher is running. Do you want to terminate it?" -> I click "ok" -> The patcher screen pops up -> I click play -> Nothing happens.
I've tried to uninstall and re-install, and I tried to run as admin. Still nothing, and I want to play because I just got back from vacation.
I got the same error, however starting it as administrator got rid of it. Have you tried the repair tool? Though, you did uninstall it...maybe some things are left in the registry that are messing with it.
Another advice would be to ask the lol official support forums for help
Does it give you an error saying you can't connect to the HTTP client or something after not opening for a while? If so, there's no real "fix" for it. I've seen a ton of people post about it in the LoL forums and I haven't seen anyone (Riot or anyone else) give anything that would really fix it. Eventually it opens up on its own, but it's still annoying as hell : /