I was in a very similar situation, I stopped studying for 2 years … (I was at level 15).
I would suggest to have a look at this thread: How to kill a giant review backlog
Personally, I started by using the self-study quizz to see how much I remembered after such a huge break. Suprinsingly, I still had a good knowledge of kanjis from level 1 to 6. For level 7 and 8, I had to do a bit more of self-study and level 9 was way too hard. So I decided to reset to level 9 
I hope this help !