display properties

I have a computer that the color was changed from 256 or something to 16, and the size of the pixels was changed from 800×600 to 640×480. Normally I would be able to change this back, but now the it won’t let me because the only color option is 16. This does not allow me to even change the size of the pixels back, basically leaving everything to big for the screen. How do I fix this?