Oh wow that sounds awful I can’t imagine how bad it must feel to lose something you’ve worked so hard on. I hope you don’t feel any pressure to rush yourself into re-writing it, but it would be best if you don’t leave it too late and get yourself into a slump. You should keep your notes in a folder or all in the same book, if you keep them online then maybe print them out for extra measures. I hope you find your motivation to continue! You can do it!

Don't give up!!! I know what this feels like, it happened to me too :c but it helped me to take another while off from the story and accept that what you did, is gone, but you can rewrite it, even write it better than you had before <3 you can do it! <3

I'm sorry to hear that cause I know it .
Once, I realized that I lost my unfinished 70 pages of novel when I was going to continue it (I wrote it during my senior high school and I just got free time to write again during the second year in uni). Felt like flipping the table.
It took me some times to accept it and move on. I didn't force myself to rewrite it cause I couldn't remember the detail either since it was done like a long time ago.
But as yours is fanfic, I'm sure there are people who are waiting for it. So take your time, calm yourself down and re-writing it when you're ready. You could write other story too if you have to distract yourself from the lost one for a while.
Good luck! :)