Are you at Hoover Dam? The final quest?
If so, you can't fast-travel anywhere, better reload a previous save before you activated the final quest or start a new character.
And before you go "Well that's bullcrap!". No, it isn't, there was a pop up right before the final quest that told you that it was the point of no return and if you failed to read it you have yourself to blame.
If you're "not" at Hoover Dam or at Legats Camp then feel free to disregard this reply.