That could be caused by any one of a number of reasons. The best thing to do is to have a pen and paper ready the next time it happens and write down the Stop error code displayed on the blue screen. In this image, the stop code is near the bottom just below "Technical information:", and reads ...STOP: 0x00000050.

http://upload.wikimedia.org/wikipedia/commons/a/a8...

Yours will likely be different, but write it down quickly, it won't be displayed very long.

Using the stop error code use these or search the web to help find the reason for the code and then you'll be able to find a solution.

In this first link you can either search through the error groups or in the above right, enter the error code and let it search for it.

http://pcsupport.about.com/od/findbyerrormessage/t...

http://aumha.org/a/stop.htm