So got into New Vegas, using the science option, look around for a bit and went back out to do other quests. Now I want to go back into New Vegas but it says I need a key, but the robot doesnt ask me anything. So I tried getting a key from Ralph but that also didn't work.
Does anyone know how to fix this?

You could kill the securitrons for their gate key. Though that would make them all hostile at you for a little bit.

Use the console.

Open console. (~ key)
Click on door.
Type: unlock

Carry on.

Or

There is another way without the console but I hope you get along with the NCR. (That's a hint.)