How do I prune an enormously overgrown lilac bush?

I live in northern Ohio. I moved into my condo in April. I have a large deck that has what I thought was a very tall skinny "tree" growing on the side of the deck between my deck and my neighbor's deck. There is only about 2 feet of space between the two decks (they have a privacy wall) and this "tree" spans across the middle of my deck between the two decks. This "tree" is about 50 feet tall and about 3 or 4 feet wide. I noticed as it started to bloom buds and then eventually leaves, that a large portion of it appeared dead. I had planned on possibly removing it eventually. Well as the time passed, I looked out my window to see that this "tree" is a Lilac Bush that never got pruned with beautiful blooms dominating the top and next to nothing below. I absolutely love lilacs. I cannot just kill it. I want to cut it back to an appropriate size of about 10 feet at the most. Is this ok to do now? Should I wait until the fall? there are branches (the bigger ones) where there are no flowers on any branches except the very top which we can't even enjoy because it's 50 feet in the air. should I just trim it all the way down to 7-10 feet and have a rather dead and ugly looking bush this summer and if I do this, will new branches emerge next year so I will have glorious blooms next year?