Stats for one site disappear after Matomo upgrade

I had some issues upgrading to the latest version of Matomo (v4.15.0). After doing an automatic upgrade through the admin panel, the Matomo site would just load as a blank page. After manually updating things seem to be back to normal except that one of the three sites in my system seems to have lost all its stats. Previously there was over 10 years worth of visitor data (mostly imported from Google a while ago, as with the other 2 sites), but now the reports for just this one site show zero visits.

Obviously I would like to get this data back! Is there anywhere obviously I should look to fix the issue?