Sorry for the trouble, Billa - I'm afraid the first poster who responded is most likely incorrect, though.
The first thing you'll want to do is try switching to the "All Items" perspective, under the menu of the same name. If you search for the items in question, do they appear there? If so, the items weren't gone, just hidden from view by the settings chosen under the View menu.
Even if you're not seeing them in the All Items perspective, OmniFocus makes periodic backups of your data, and by default it backs up your data twice a day, plus when the app quits. Your backups are stored in your Documents folder, in a folder named "OmniFocus Backups". Just double-click the most recent backup to open it up alongside your current database to compare the two. If you find the missing items, you can just drag them from the backup and re-add them to your "real" database.
Even if that doesn't work, you can email our
support folks and we're happy to investigate this with you. We haven't had any bugs like this in a long, long time, but it's always a possibility. Sorry again for the trouble!