64 bit not showing up in list of bit resolutions

I have a new Asus Duo t6600 4GB memory 320GB HD Windows 7 laptop
and to start the 64 bit option was there and i play diablo so i have to run in 16 bit
with 600x800 res and have been switching back and forth 64/16 and one day i go to switch and 64bit option was gone PLEASE IF YOU KNOW HOW TO GET IT TO SHOW UP LET ME KNOW THANK YOU SO MUCH

The ability to switch screen resolutions and bit color depth is a function of the driver for your graphics card. Reinstall the driver with preferably the most current version from the manufacturer's website.

Ok you are the man/women I did not think of doing that I will post again to let you know the if that does the trick thank you for your wisdom and time