SharePoint 2013 I have a subsite where all users lost access upon their removal from the top-site's permissions list. Re-adding them to the top-site's permissions does not grant them access back to their sub-site.
How might I go about restoring access to a group of users to said subsite in this situation? Any help would be appreciated.
Sounds like all the subsites had custom permissions with custom groups.
Re-adding to the top-site group is not enough
Goto /_layouts/15/groups.aspx of the SiteCollection and check if all those subsites had groups
If so
This is unfortunate, but common. What probably happened is you had a broken inheritance with this sub-site. When inheritance is enabled - you lose those changes and changing it back does not restore your groups.
The only to restore it is by restoring from a backup, or manually recreating them as pointed out by @DannyEngelman
