Hi
First off when you connect the external monitor do you press the relevant key combination to enable the monitor, on most laptops its the FN key (bottom row on the left) and then one of the "F" keys, on my acer its F5, on my other dell its F8.
As for the white screen this maybe caused by the screen invertor failing or the screen itself failing, most invertors (inverters) can be bought for about fifteen pounds from ebay, screens however can cost upwards of £60.
It could also be a loose connection and would require the stripping down of the laptop to get to the ribbon connector that joins the screen to the base unit.