I had researched several guides and forum posts on this quest "disappearing" after a certain amount of time which it had in my game. I tried everything but the quest would not trigger.
After completing the last main game quest "Take It Back," I fast traveled to the Trainyard location where you go into The Pitt DLC. Once The Pitt location loaded, I went back into the Capital Wasteland and fast traveled to Vault 101.
The emergency signal from Amata with the password to open the vault became available and allowed me to complete the quest.
I haven't confirmed that you have to go through the main story questline and then to The Pitt to trigger the signal again but it seemed to me going to The Pitt reloaded the Capital Wasteland map and made the quest work again.
This does confirm that the quest isn't lost forever and by traveling to the Trainyard and going through to The Pitt DLC and back to the Capital Wasteland should allow the signal to be re-established.