I'm sorry for the trouble this is causing!
You shouldn't need to reinstall, though. I know the support ninjas have gotten a couple of reports like this, but I can't remember the specific cause of the issue. If you call (800.316.6664) or
email them, they should be able to help.
Steps for rebooting the app vary a bit depending on which iPhone you have. On a 3gs or an iPhone 4, double-press the home button to bring up the multitasking tray, then tap-and-hold OmniFocus' icon until the red "quit app" dot appears. Tap it and the app will be force-quit.
On an earlier iPhone, the app shuts down fully every time you switch to another app.