@Thijs11
Rule #1 never, ever cancel a guest, even at their request, you will always be penalized including having the dates blocked etc. And never click 'yes' if you get an email from Arbnb asking you if you agree to cancel, it is interpreted as you canceling and you will be penalized.
https://www.airbnb.com/help/article/990/i-m-a-host--what-penalties-apply-if-i-need-to-cancel-a-reser...
I suggest you call Airbnb and see if they will reverse the blocked dates and all other penalties associated with cancelations, it being a rookie mistake. Would be good if you have some documentation from the guests stating their wish to cancel and asking you to cancel because of an injury.
For future tell the guest to cancel their end, with documentation they can claim extenuating circumstances. Good luck!
If this is the canceled reservation (see screengrab) it is on your review page, and if Airbnb will not delete it, you can respond to it saying the guests asked you to cancel because of an injury, so that future guests do not think you are in the habit of doing this.