You can try to contact staff and hope for the best... If that doesn't work, keep an eye out for the wiki, if the user it's inactive for 60 days, you can be made bureaucrat again in 1 week if you follow the adoption guidelines.
I saw you contacted him on his wall, and your approach wasn't the best, all caps and I quote "WHO MADE YOU ADMIN?! AND WHAT HAVE YOU DONE TO MY WIKI!!!!!!!!!!!!!!!!!!!!", etc it is not encouraging productive communication and mutual respect, maybe you can try introducing yourself, explaining why you were away, how much the wiki means to you so you still want to be bureaucrat (I would suggest admin for the time being, maybe later he can promote you or you can adopt if he dissapears), and what you plan to do with those administrative rights, how you plan to better the wiki, if not that is only a right you have but you are not using it, he is not likely to give in to your request, you don't need administrative rights to contribute to a wiki on a subject you love.
If you don't have time for it, maybe it's for the best, to prevent the situation from happening again on other wikis you admin, better to login at least once a month, and make an edit, even a small one (clean up spam, answer comments, post a discussion post/poll, add affiliates, add WAM log, etc just edit something)