I have had this avacado growing from seed, perhaps 3 or 4 years now. It has always grown well. I could leave it in the window for 2 weeks and at worse, it would wilt badly. With water, it would be good as new. Before being left alone, it was well watered with the blinds closed but this time it was left for 4 weeks, with apparently dire results. The stem, with bark on the bottom 20 cm of the ca. 1m tall plant, looks fine, but everything at the top looks dry and shriveled. The question, what to do now?
Scrape back a little of the surface of the main stem with a fingernail, starting near the top, until you find an area that isn't dry and brown inside, but moist and slightly greenish looking, and cut back to that point. If its not dry and brown inside even at the top, just trim off the dead growth. Water thoroughly, keep watered ongoing.
If you haven't repotted during the last couple of years, it may need a bigger pot to accommodate its roots - I can't say whether it does or not because the pot is not visible in the photo.