I currently have a Mac Desktop Pro, and I can hear the fan, but nothing shows up on the monitor?? Help me please?

I can hear the fan running extremley loud, but when I turn on the monitor, it shows up as a dark screen. We have a group at school that delivers school news throughout the TVs in our school and we have been using a really old iMac g3 and we would like to use the Mac Pro, but it will not show anything on the monitor. Please Help Us!