The Ward Website Administrator should be the person to look at it. With the exception of any private calendars that he didn't created himself and/or didn't have specific edit permissions given to him by the person who created it, he should have admin rights to all ward calendars and can assign permission back to anyone that has been removed, even if he isn't explicitly listed on the calendar as having edit permission. At the very least he should be able to go to Settings -> Calendars, and see the calendar there in the list, and identify whether it is a private calendar or can be seen by the entire unit. (In addition to the Ward Website Admin, ward clerks, the executive secretary, and bishopric members are default administrators with those same permissions.)
If the calendar itself was deleted by mistake, rather than just the YW secretary's permissions having been removed, it goes into a "Deleted Calendars" view that can be found under Settings -> Calendars, and it can be restored from there.
The other thing that could have happened to change her permission on the calendar is a change in MLS (whether on purpose or by mistake). If her calendar permissions were granted by calling and not individual and here calling in MLS was changed from the standard YW Secretary calling, that will remove her permission as well (generally the best way to go with permissions is by calling so that when callings change, permissions are automatically given to the newly called individuals)