I want to go back to Wanikani after years of not touching Japanese at all.
I have a lot of flashcards I’ve already mastered, and some that I left halfway through… Right now, I don’t know where to go from here. I’m level 12.
Has anyone else been through the same thing? Any advice?
Welcome back! 
I’ve been falling and climbing back for a long time.
Took me 8 years to reach level 60, but I did it.
My advice is to start moving. If you can just start doing reviews and lessons without reset – great.
If you can’t – do a reset. Not necessarily to level 1 (although this is also an option), but to the level you feel comfortable with and start doing reviews and lessons again. The important thing is to restart daily studying.
In any case, best of luck with your studies! 
Welcome back!
At level 12 you could probably still slowly clear your review pile and go on from there, but my piece of advice is: don’t be afraid to reset a few levels (or all) if you feel like you don’t remember too many things or it feels overwhelming.
I did WK all the way to level 53 in 2022 and then didn’t study Japanese for almost 4 years, so when I came back around a month ago I decided to do a full reset and start over again. A bit drastic perhaps, but in this way I’ve been able to get back into the learning habit and building a daily routine. I remember almost everything from the first few levels so reaching level 5 only took me a month (this is not even max speed), but the refresher is also nice and I can use the “free” time to also get back into reading and to fix the scripts that broke while I was away.
I would give your reviews pile a go first and see how that goes before deciding!
I was up to level 40 when I stopped. I thought it was only about 3 years ago, but now I’m realizing it was more like 5 or 6. I knew it would be painful turning off vacation mode since I really didn’t remember anything, so I went ahead and reset back to zero.
I believe it was the right decision for me as the discouragement and frustration of reviewing stuff that was completely gone from my head would have been too much and I would have dropped it again.
Starting over it’s definitely easier and faster, but it seems I have mostly the same leaches 
. Another good reason to restart is a lot has changed since I went away: kanji moved to different levels, new vocabulary, new radicals, new mnemonics. I’m finding the changes mostly positive, but would have been awkward encountering them coming back at a higher level.