Nikon D5000 aperture saying Lo?

Ok, so I can be in a very well lit room, have the shutter speed set to 1/100 and the ISO at the highest and the aperture will say Lo. No matter what I do. I have no idea what is wrong, or if this is normal or what, but it just seems like its wrong… Especially since my shutter speed has the option to go up to 1/4000

Well, the camera says there's not enough light. Either you left the lenscap on or there's not enough light

Slow down your shutter speed until it quits saying lo.