I awoke to an email from Grammarly saying that my password had been successfully updated and my email address changed too. And I should email support right away if it wasn't me who made the changes.

So I did. I got my account back, but the hacker had deleted all of my files on the Grammarly servers and left a half a dozen of his. Long story short, Grammarly said there is no backup of my files. I was a premium user up until this. I've canceled my account and there is no prorated refund.
I had about twenty files on their servers, most of them from the last book I wrote, so my loss wasn't catastrophic, just enlighting. The password used for Grammarly was a couple of letters and six digits. I guess from now on the lesson is to use passwords that are long and complicated like those generated using a service like Lastpass.
So if you're using Grammarly and your password is something like jt010161 you might want to go change it to something with upper and lower case letters and symbols which is a lot harder to crack, like Jt^9Hl#uMs13R5 for example.
The lesson for me is to change all my passwords to something strong. I never really thought about Grammarly being a target for a hacker, but obviously, I was wrong. Don't be me, go change your password now.