When I opened GTA V, my monitor blacked out and a pop up showed saying:
Not optimum mode. Recommended mode 1440 x 900 60Ghz
What to do? How do I fix this?
It's just suggesting a resolution setting for you. You don't really say whether it gives you the option to change it at that moment or what monitor you have etc., but you should be able change your resolution and run the game just fine.
Even if it's not the optimum resolution, it's possible that you can still play the game in a less-than-optimum setting, it just won't look "perfect".