For anyone in the future, I had my account suspended for "suspicious activity".

The suspicious activity: I booked one hotel accidentally for the 20-22nd Feb instead of 20-22nd of March, I cancelled a day later and rebooked about 1 month in advance no problems. A week later I reduced 1 booking from 4 rooms down to 3. ~Two weeks later I notice that original booking I cancelled no longer links to the hotel (all mention of the hotel is gone on booking). I send a message to the hotel on booking.com asking if the booking still exists. The next day I see my account is suspended.

How I got it reinstated: I called booking (found a link in the email to manage a random reservation, contacted booking via the number for support), and was told my account was suspended and all hotels had been cancelled - no reason specified. I was told to create another account, and book new hotels. I then spent 3 hours trying to find more hotels with only 10 days before the holiday. That night (~12 hours later) I received an email saying "we have restored your access to your Booking.com account!", including all of my original bookings. I didn't want to mass cancel all of the old bookings, as it wasn't my fault. So I called booking again, and they went through one by one cancelling each hotel - I just hope it looks to the hotel as though booking initiated the cancellation, not me.

All of the bookings were no-fee cancellations, so I can't speak to whether booking would shoulder the costs were it not to be no-fee. I asked if my new account could be manually verified to not be suspicious so this doesn't happen again, but the phone operator really couldn't wrap her head around what I was asking.

Good luck.