Defragging won't cause the screen to go white. It would either be an error with the driver, the GPU is dead or the display is in need of replacing.
You need to power it off. If necessary you will have to either pull the battery, or if you can't/don't feel comfortable doing so, then you will have to wait for it to run out of power.
Once that is done, let it charge for a short while (if you had to run it down) or reinsert the battery. Try then to turn it on and see if you get the same issue. If so, you may then want to try connecting an external monitor to see if you can see on it. If you can, then the attached display is likely the problem. If you can't, then the GPU is more likely the issue.
NOTE: If the laptop is under warranty...