[HANGAR]

Are these incandescent lamps or LED? Are they wired to a switch that is (on/off) only, or can they dimmed? Are these lamps running on the 12VDC or 120VAC circuit? If they're incandescent running on 12VDC, then it could be a low battery issue like hankaye said. If these lights are connected to a dimmer-switch, the dimmer itself might be fried and needs to be replaced. My suggestion to start: Replace one of the lamps with a new one and see what happens. If it's as bright as it should be, then replace all of them with new lamps. If the new one is still dim, then you know that you have other issues and need to trouble-shoot a little more.