After hitting the launch button nothing would happen so I tried relaunching it, but it said it was already running and a black box appeared at the upper left hand corner of my screen. I could minimize and restore it, but nothing happened. I had to restart my computer to exit the application (when I used task manager to close it and tried relaunching it said it was already running). Running it on Windows Vista 64-bit if that helps.
wanderix2 votessame issue here. 
Gank2 votesSame issue as well. Windows 10 here.
I press the button and nothing happens. I open the client again, says another instance is running and it just shows a dark blue/black square after that.
Gank2 votesIt's a problem with the launcher. If you go to your installation folder of LoL and open LeagueClient.exe directly there is no problem.
Augment SPEAR1 votesI tried opening it once normally and once as administrator still running into the same issue sadly.
Gank1 votesIt doesn't work if you already opened it once with the launcher. You need to open task manager and close all instances or restart PC first.
Augment SPEAR1 votesI did, and tried it a second time as well. Thanks for tips though. I'm going to uninstall and try to install the legacy client.
BladeSteel1 votesThey said theres a bug and they'll fix it. So to get back to the old client got to c drive -> riot games -> league of legends - > RADS - > system -> and open user.cfg with notepad. Change "leagueClientOptIn = yes" to "leagueClientOptIn = no" and you'll be back on the old client until they fix it.
kaeric1 votesthannk buddy it work for me (love you)
RecessHero1 votesSame problem here. I tried the leagueclient.exe work around, same problem happened.
Edit: nevermind. I clicked the wrong the thing. The leagueclient.exe work around does work.
the weeaboo1 votesThe real question is why the fuck you are still using Windows vista
Bedins1 votessame problem here, if i launch lolclient.exe it shows me directly the darkblue screen
a faNATic1 votesI had the problem too (got a blue square in the corner in the corner, file wouldn't open, tried going to installation folder, etc.). Managed to get it to work by going to task manager, closing the application if there is one. Then went to the processes in the next tab of task manager and ended the LeagueClient process. After that, right click league --> properties --> Open file location --> LeagueClient (the one with the new updated "L" symbol). Hope this helps!
BaBa YeGGa1 votessame here..
ShadowWillOwnYou1 votesyea same here.