Occasionally the Wiki is hit by spammers trying to increase Google's ranking for their sites by adding links to them. These spammers may replace a page's content with dozens of links, or they may insert links into the existing text.
Fixing the Changes
To fix changes, follow the little 'i' icon next to the page's RecentChanges entry, or select Info on the page itself (which may be an 'i' icon depending on which theme you are using). You'll find a history of all versions and a revert button for each of them. Usually you can just revert to the previous version. You can also select a version and then on the page (possibly depending on the theme) choose Revert to this revision from the actions menu.
Preventing Future Attacks
Generally, the TextCHA support should help prevent attacks and reduce the likelihood of recurrence. Previously, the following advice was much more important.
Edit the LocalBadContent page. This page records a bunch of regular expression patterns that are applied to all of the links in a page; if a matching link is found when the page content is saved, the save will be disallowed and an error will be reported.
Remember that this page lists regular expressions, not strings, so the '.' character will match any character. If you're matching a domain such as foo.com, add foo\.com to the page.
See AntiSpamGlobalSolution for more details.