Why do I lose signal to my second monitor when I disconnect the first?

I have an old custom built gaming desktop, has a nice big monitor I want to use with a newer laptop I have when I’m home. I have an old smaller monitor I want to use with the pc. When they are both plugged in, they both function fine, but when the big monitor is disconnected, the smaller one loses signal every time. If I shut down the computer, and disconnect the large monitor, the small one works during the load up process, but loses signal as soon as it finishes the start up. Pc was built by a guy I don’t really talk to anymore, don’t know if he made some weird setting change or something I don’t know about or understand. Any advice on how I might resolve this? Thanks, I would appreciate any help that can be offered.

amd phenom II 3.20 ghz

ati radeon hd 5700

4 gb ram

running on windows 7