After a battle, the program crashed and asked me to send a report. Since then, I'm able to match a team, but cannot open the play screen any more. In Active Monitor, the Userkernel is not responding.
Here is a solution, this works to my case, may be not applicable for other problems that cause userkernal not responding.
1. Open Terminal.
2. enter "rm -r ~/Library/Application\ Support/LolClient" and press return button.
enter "rm -r ~/Library/Caches/LolClient" and press return button.
3. Reopen LOL it should be work.
These commands just clear caches and some configuration of LOL, so it is safe. Alternatively, you can open the path with Finder and delete the folders there.