If your monitor will only display is safe mode, it's probably because your computer's resolution is beyond the ability of the monitor.
If I understand you correctly, your computer is configured to a resolution setting compatible with your old monitor. Your new monitor, being used, is most likely an older model that can't support the resolution settings of your system, and so nothing is showing up. Safe Mode, I believe, runs at a standard resolution that is lower than your normal one so your monitor can still support Safe Mode graphics. If I'm correct, you need to change your resolution settings under Safe Mode to a lower one and you should be able to run Windows normally.
Of course I may be entirely wrong