After a moment to shut down, the iPhone will have completely powered off. Again, press and hold the sleep button at top-right until the iPhone starts up. Now open Safari and see where you are.
How to restore lost Safari bookmarks to your iOS 6 iPhone: Switch off iCloud syncing
Some users report that the problem seems to happen more often when they have ticked the option for Safari to sync its bookmarks with iCloud - the theory seems to be that bookmarks become inaccessible during the sync process. We haven't found this terribly helpful, but if you want to try everything, head to the Settings app, then scroll down and tap iCloud, then slide the Safari toggle from ON to OFF.
How to restore lost Safari bookmarks to your iOS 6 iPhone: Clear Safari's cache
One last solution that some users swear by is to clear Safari's cache. Go to Settings again, scroll down and tap Safari. Now tap 'Clear Cookies and Data'. Bear in mind that this some cookies are helpful, and you may find that some websites you visit regularly (and allow you to customise the browsing experience) have reverted back to the default settings; other autofills and similar may also need to be entered again.
Sign up for CIO Asia eNewsletters.