Loose Video Cable Connection
Always check the video cable connection first. Reconneting the video cable may fix your problem.
If reconnecting the video cable does not help, most likely the problem is either with the monitor or with the graphic card.
Defective Monitor Screen
Check if the monitor screen is causing the problem, by testing your computer with another working monitor screen.
Disconnect the video cable from the monitor connector. After that you connect another working screen and test video display.
Defective Graphics Card
The procedure is the same with checking a defective monitor. If it still displays a white screen, most likely the problem is with the graphics card and needs to be replaced.
NOTE: In most cases, a white screen error is fix without doing anything, just let your computer rest in a few days (2 - 3 days). So you may want to consider this first, before buying a replace.