Try shaking it up and keeping it around 75F. Most likely the yeast settled & went dormant. If that fails, get a packet of dry brewing yeast. Bread yeast bites for fermentation, but it MIGHT work for carbonation. It is bred for fast CO2 generation and eating simple sugars.
If you go that route, let us know how it works.
Remember one unassailable statistic, as explained by the late, great George Carlin: "Just think of how stupid the average person is, and then realize half of them are even stupider!"
"I would like to die on Mars, just not on impact." Elon Musk