If you have Wikis, but you are no longer having access to change your wikis, also the "Manage Wiki" option is simply not visible in your account for anybody, including administrators, the issue might be related to your subscription.
On-Demand accounts using older pricing models have the option to choose optional modules individually when renewing their subscription. One of these optional modules is the Wiki. It might have happened that during renewal the Wikis were removed from the optional modules list.
The solution is to change your subscription:
- either, include the wiki as an optional module
- either, upgrade your subscription to use the latest pricing model (and latest version), which includes all the optional modules.