Flat Screen Monitor problems

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My Dell Flat Screen Monitor has been going white on my every once in a while. At fisrt it appeared not working so my stop mom was gonna throw it out, but she just left it in the basement. I read up on how to fix some monitors and i thought "why not fix the one in my basement" So i brought it up, plugged it in, and it worked fine! But then a couple hours later it went white! I smacked it and it came right back on! Every so often it goes white but not often enough to throw away! Anyone know what might be causing this and possibly how to fix it?