Method 2 suggested by Azam would be a good place to Start.
More information is always helpful when looking for a solution.
Have a look in the System and Application logs in Event Viewer for Errors and Warnings and post copies here. Don't post any from than more than 48 hours ago.
You can access Event Viewer by selecting Start, Control Panel, Administrative Tools, and Event Viewer. When researching the meaning of the error, information regarding Event ID, Source and Description are important.
A tip for posting copies of Error Reports! Run Event Viewer and double click on the error you want to copy. You will see a button resembling two pages. Click the button and close Event Viewer. This places a copy of the report into your Clipboard. Paste into the body your message. Make sure that this is the first paste after exiting from Event Viewer.
Hope this helps, Gerry Cornell