Uh, this is your first post, so you get some leeway...but, unless you provide enough info to help you troubleshoot, it's a guessing game and that isn't what the forums are all about...
What LR version?
What OS and version?
What computer?
What are the specs?
Is this a new problem or have you had problems in the past?
Any recent changes to your computer?
Lot's of info you are not providing...