I have had 2 cell phones that got the "white screen of death": a Motorola and a Nokia. Both were on the Cingular network. I had to send them in for replacements.
No one knows what causes the "white screen of death". I don't think anyone really cares. It's easier to replace the phone than to figure out what causes it.
You can do the some or all of the following in various combinations and orders (but they usually do not work):
1. turn the phone off and then turn it back on
2. charge the phone's battery
3. remove the battery, wait 10 minutes, put the battery back in
4. inspect the electrical contacts of the cell phone, battery and/or SIM card and see if they need to be cleaned.
5. replace the battery with a new battery
6. get a new SIM card
7. flash the phone's ROM
Google "white screen" "cell phone" and you get 83,300 hits. So, it's not like it happens that often. I have one theory about what may cause a cell phone to mysteriously stop working: leaving it in a hot car where the temperature may get up to 200 degrees - especialy if the cell phone is plugged into its charger.
If you call the cell phone manufaturer's tech support they will probably walk you through items 1 through 4 from above. If those 4 things don't work, they will probably tell you it is not fixable. If they can get you to say one of the following:
1. The cell phone was dropped
2. The cell phone was exposed to moisture (it got wet)
3. The cell phone was exposed to very hot or very cold temperature ("very" is an undefined amount of temperature)
4. You attached the phone to a computer (this is a new question they have started asking)
They the will probably not honor the warranty. If you have insurance (almost always with at least a $50 deductable) it will cover those 4 situations.