Welcome to the Pro Tools forum.
Before we can help you out, we're going to need a little more information about your system, how it is set up and how you have it optimized. Please take a moment to read the
Help Us Help You thread, which has good information about basic Pro Tools setup and troubleshooting. There is also a section explaining how to post a Sandra report, which tells us a lot about your system.
Also make sure your computer has at least one ASIO compatible audio device. Without an audio device, Pro Tools probably won't start properly.