I'm afraid it won't be possible to re-attach the broken top -- this is the wrong time of the year to attempt such a technique, but the good news is that this is a good time of the year for the avocado to grow new shoots that will turn in to sturdy branches that ultimately, will help it look better.
Most people with seed-grown avocados get attached to the seedling and wait too long to pinch and prune, so you might say the wind did you a favor.
I can see at least three bumps along the bottom "trunk" that could potentially grow out into new shoots just below the break but far enough down that when you clip off the jagged break for a clean cut that won't invite disease or pests, you will still have a bit of stem above the buds.
At this point, having lost all those leaves, this plant will not go through as much water as before so be sure not to overwater it but carefully monitor the soil moisture level so it doesn't dry out in the summer heat. Clean up the break with a pair of sharp pruners with a 45Â° cut that leaves extra stem above the bud you want to grow, and let it rest for a couple of days... You might actually take this opportunity to uppot to a container that is one size larger at least or repot with fresh potting mix -- this will give it an infusion of fresh fertilizer that will help it grow. What dimension pot is it in now? Avocado seedling should be in at least 6" diameter by 6" deep pot.
With no leaves, the bark might sun burn. You may need to move it to a slight ly less sunny spot in the beginning.