system very slow and i cant get into my singleplayer worlds

When i first got the update 1.8 my pc was really slow and im pretty sure its because of the update i thought it was just because it was new and still a good amount of time after it was still slow. Now when i go to singleplayer to go to one of my worlds i will click on the name and i will try to hit the "play selected world" button it wont put me into the world and i cant click anything else but i can move my mouse. Im not sure if its just that my pc is slow or what if there is anything you know that might help please tell me.