Monitor's native resolution is not an option in the display preference

I've got two ViewSonic monitors hooked up to my G5 at work. Yesterday they were working fine. I rearranged my office and so unplugged both monitors. I only moved the desk about a foot, and it's on casters, so I don't think there was any physical damage to the monitor. However, when I hooked them back up, only one of them worked. I switched the cables. One hooks up to the DVI port and the other to a port called D-SUB (sorry, I don't know what that is). The one hooked up through the DVI works fine, but the other keeps flashing a message telling me "for best picture quality change resolution to 1680x1050" but when I open the display preferences, 1680x1050 is not one of the options. I've tried restarting. I don't know if there's a preference file corrupted, but I wouldn't know which one it is. I think it's weird that one monitor works fine but the other doesn't. The second monitor doesn't work at all (I get a 'no signal") message when it's hooked up through the DVI port.