Though you've probably made a good site optimization before, SE changes their algorithms on the run, so the reasons for your pagerank drop may be different and not obvious at all.
But you can try these basic steps to fix the problem:
1. Check the SERP for your keywords, maybe other sites also dropped positions in search results. It was noticed that sometimes Google tests its algorithms which may result in such ranking drops. In this case all you need to do is just wait for a few days.
2. Check if there was server overload. Test your page speed. Maybe there is a technical problem.
3. Check the parity between keywords used in your content and the expectations of your site's visitors. It's known issue that Google may lower site's rank if the intent of the site do not respond to it's keywords.
4. Check competitors, maybe drop is caused by active actions from them. In this case you need some efforts in order to gain your rank back.
Cheers,
Tara.