TOPIC: Help Please

My Garrys Mod won't work does anyone know how to fix it.

What do you mean by it's not working

Can you be more specific?

Is it not loading up? Is it constantly crashing?

When I open it up instead of opening up full screen it shows a green box then it says engine error no steam user and I've tried reloging

Have you tried any of the following:
Restarted steam
Verified the integrity of the game
Googled the issue?