SourceMonitor v3.5.16.62
I replaced a dying old monitor with a new one with a higher resolution and after restarting SourceMonitor, it was never shown on the but it was running as seen in the task manager.
The problem is in the registry HKEY_CURRENT_USER/SOFTWARE/Campwood/Setting/FramePosition, the value was -1500,50,1286,737 and this is causing the window to be outside the current view.
Replacing the '-1500' with 0 fixed the problem.