'Read only' error message when trying to open Anki

Whenever I try to open Anki on my desktop, the following error message pops up:
‘Anki could not create its data folder. Please select the File Locations section of the manual, and ensure that the location is not read-only’.
I have never seen this message before, and I don’t know why it’s suddenly happening - yesterday Anki was working just fine!
I have tried uninstalling and reinstalling the Anki application, but to no avail. When I log on to AnkiWeb or AnkiDroid, I can see that my most recently created cards are not there and so have not been synced - I really don’t want to lose these cards… Please help!

Please see https://faqs.ankiweb.net/windows-permission-problems.html