a1200 on a 1084s?

[Matt_H]

The most likely problem is that the screenmode is set to something other than NTSC, PAL, or Euro36. Can you get to the early boot menu (hold both both mouse buttons when booting)? If that's so, then boot without a startup-sequence and

delete sys:prefs/env-archive/sys/screenmode.prefs

Reboot and you should be in a basic, default screenmode.

[Matt_H]

What sort of cable are you using to connect the 1200 to the monitor?

There must be another program screwing with the display. Can you report back what's in your WBStartup drawer?