Could be either over/under watering, or a cold snap...... You need to keep them evenly moist and they can stand 1 or 2 days dry, but no longer.
I have a Mandevilla (is that the kind you're talking about?) and it needs watering 3x a week - thoroughly.... I have 3 new buds on it (previously only 1 flower, but I planted it late, so it's recovered now).
You woudn't be talking about a Morning Glory, would you?
There are so many types of "trumpet vines" I think.....