The worst thing that could happen would be a lot of hum and a weak signal.
The typical balanced input jacks on a commercial power amp will also accept unbalanced signals. Of the two signal leads going in, ground one and apply the unbalanced signal to the remaining pin.
I always have some XLR to 1/4" adaptors in the kit. I keep a pair wired pin 2 hot and pin 3 ground and another pair wired pin 3 hot and pin 2 grounded.
WHich ones get grounded usually don't matter, unless you are driving a system with other amps and phase matters.
Watch the levels though, you didn't state what you are connecting. The hot line out from a mixer/receiver might overdrive a mic input on something else.