If you encountered the Fatal error: Call to a member function isLockExists() after trying re-index your Magento site, then don’t panic. It’s just a minor glitch after upgrading from 188.8.131.52 to 1.9.1. Just clear your cache then try re-indexing again. It will work afterwards. If your latest Magento site still doesn’t work, try logging out the logging in back again and all the errors will be gone in no time.