SP User groups have disappeared from site

My '<site name> owners', '<site name> members' and '<site name> visitors' groups are no longer appearing on the site permissions page of my site collection.

This came to my attention because users were reaching AccessDenied.aspx when trying to access the site because 'NT AUTHORITY\authenticated users' were no longer granted access through the '<site name> visitors' group. Once I added them to the 'Viewers' group everything was fine again.

I went and queried the content database along the lines of:

SELECT Groups.Title, UserInfo.tp_Login, UserInfo.tp_Title, UserInfo.tp_Email
FROM GroupMembership with (nolock)
	inner join Groups with (nolock)  ON GroupMembership.GroupId = Groups.ID
	inner join UserInfo with (nolock)  ON GroupMembership.MemberId = UserInfo.tp_ID
where 	userinfo.tp_isactive = '1'
	and [GroupId] = 3 or [GroupId] = 4 or [GroupId] = 5

(3, 4 & 5 are the IDs of the groups in question) and sure enough I can see the groups and their members but even after a reset of the WFE hosting central administration they no longer appear on the site.

Any ideas how I can get them back?

Thanks