What symptoms of a hangover are you talking about? If you're speaking of anxiety, that's most likely not a hangover, but rather the typical after effects of alcohol consumption.
Taking Ativan for hangover symptoms would technically considered abuse of your prescription, since that isn't what it's prescribed for, and all around a bad idea.
If you struggle with anxiety, I would highly recommend laying off the alcohol, completely is best, but in the least, do not drink to the point of intoxication, like one or two drinks at best. And, don't mix the Ativan and alcohol either.
I'm not sure if it does or not but I highly recommend you do not take Ativan while drinking or when you may still have alcohol in your system.