Dell Optiplex 755 desktop won't go past post

I took the battery out of my Optiplex 755 dell desktop, put it back in, and now the computer won't go past post.  We have taken out the Ram one at a time, tried them separately, tested memory, is all good, but it shuts down as soon as it tries to load the hard drive.  The  hard drive works on another computer.  We will look into the graphics later, why would taking out the battery cause it to behave this way?

Thanks for the help before hand.