[GAMEPLAY] Game freezes after a few seconds after start.

Today I have experienced quite strange bug. I went into lobby, queued and chosen champion smoothly. Even the game loaded nicely. But after a start I leveled up my champion, bought items and then I couldn't move. All I had to do was closing whole client and game. After that, everything was fine. I am using Win 8.1 OS, computer is completely fine (for games too) and internet connection is stable. This bug appears at every todays game. I have tried Hextech Repair Tool and Troubleshooting from client. Any ideas how to fix such a problem?