There could be something in your .htaccess file that shouldn't be there, sending search engines in one direction, but letting humans see what they expect to see.
Or even PHP code within the page itself that checks for certain user agents or referrers and bases the page output on that.
Even once you narrow that down, you still need to figure out how it happened. The first step is usually making sure all scripts, plug-ins, etc... are up to date.
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