Whenever I slide the phone closed and then slide it open the screen goes completely white. How do I fix this?
there is nothing wrong with your screen, the problem is from the flex that connects your screen to the phone's main board. This flex must have worn out that's why your screen is not displaying any picture.
The problem is common with flip and slide phones. The flex gets worn out as a result of constant opening and close of the flip or slide phone.
I would urge you to replace the flex and I am assuring you that your screen will display picture again. You might also be lucky to find out that the flex is not well fix, that is, it may have loosed contact.
If you cannot change the flex yourself, maybe you should take the phone to a local repairer within your vicinity, so that the flex can be fixed/replaced.