Greetings and thanks for the question, there are various reasons why this happens. Without having your computer here to see what is going on is anybody s guess on how to repair it. If I may offer some suggestions. Since I also do not know what operating system you are using I will assume it is Win 7.
I would personally go to control panel and check my power options to make sure I do not have the settings set you close display or sleep mode or anything set to soon to close your machine down. Often times people have these set to go off there are defaulted usually to save battery usage and your lap top from getting to hot and shutting it down. The thermal switch may be activating sooner than normal. What some folks do is purchase one of those underneath fan coolers to keep their lap tops from getting to hot and constantly shutting down.
This can be the case have this checked out at least set the lap top for more time before closing down or turning the monitor off. Let us hope it is not more serious than appears. Like a lamp in the monitor going out. If it is new the lap top I would not worry about that issue because it is intermittent. Which leads me to believe it is a thermal or a shut down issue. Get the cooler fan for your lap top I recommend anyone that reads this post to also do this as well. Clean your lap top blow them out it is critical to do this to keep dust away and keep it clean.
Now there is recovery software which is free to download Click Here
to check this out to retrieve lost files. Also important to get those precious memories separate off your computer and on to separate CD or DVD or storage USB stick so that when this type of thing happens you have not lost your precious memories. I hope this helps you. John