Desktop will no longer see any monitor

I brought my laptop home from school and unplugged the
monitor hooked up to my family's Compaq Presario desktop
computer. After the switch, I made some adjustments so
my laptop image would fit correctly on the monitor. I
used the laptop and monitor without any problems for
several hours. I then plugged the monitor back into the
desktop and it didn't recognize it. Nothing came up on
the screen. I have done several reboots through the
keyboard and still nothing. It boots fully and
everything else seems to work (optical mouse, keyboard,
printer), but I can't see anything from my desktop on the
monitor. I also tried an older monitor that we had used
with the desktop earlier and it didn't see that monitor
either. Any help would be appreciated. Thank you.

Laptop - Compaq Presario 2145us w/ Radeon graphics, AMD
processor, Win XP Home
Desktop - Compaq Presario 5000 w/ Radeon graphics,
Pentium III, Win XP Home upgrade from Win 98
Monitors - Compaq MV940, and Compaq 1525