If you managed to set the video to something out of range for the monitor, the normal fix would be to start the machine in Safe mode
(tap the F8 key repeatedly while the machine is starting up). That'll start Windows in a mode that works with most monitors.
In Safe mode, change the monitor settings to something that's in range for the monitor. (If you don't know, use 800X600 at 60 Hz.)
Restart it into normal Windows.
Change the resolution
to a normal value.
(Question: go to the Display control panel, Adjust Resolution button, Advanced Settings button, Monitor tab. Is the "Hide modes that this monitor cannot display" box checked? It should be.)