I'd try this:
Find the small, round battery on your motherboard. Pull it out for about five minutes, and reinsert it in the same way it was to begin with. That should reset your BIOS - if changing your voltage selection somehow made your BIOS storage do something screwy, it may have messed up some of your critical settings. Be careful, though, as you'll have to reset all of your options once you do this. "Auto" is probably the default, and will do the trick for mostly everything except the date and time in a Dell system, so it shouldn't really be that hard.
I disagree with a lot of the above posters: I've changed my input selection back and forth on one of my older PCs, and it never hurt it. I highly, highly doubt it caused any serious damage, especially if it was turned off. Something probably just got messed up somewhere...
I did what you said and also installed new ram and the problems were fixed. thanks!