No Need to reboot !! (rebooting is for newbies xD but yes,rebooting the game will fix it)
My Solution : (No Need To Reboot :P)
1)press Ctrl+Alt+Del
2)go to "processus" tab
3)Search for "GameMon.des"
4)Select it and select "delete the processus"
Normally this should fix it without rebooting (well you do my solution after you close the game) , in case this didnt work for you......then reboot xD