If "Wiki Active" is grayed out before and during the link disappearing, this is the expected behavior. Once the checkbox comes back online, the wiki should too. This usually happens when you change settings, and usually resolves after background processing the setting change completes.
If this recurs for any other reason and does not clear up on its own, please report this and allow us to look at your site before you take any actions to workaround the problem. Unfortunately as the link is currently appearing, there's no evidence of the issue or what may have caused it. I can only assume that it was a normal occurrence like the one described above.