I’m sorry for the trouble here! Often, tasks seem like they “go missing” because of OmniFocus’s View settings. If you’re looking at a project with only deferred tasks, for example, and you switch the View setting to Available, nothing will show up. You can check your View settings by clicking the View icon (which looks like an eyeball) in the OmniFocus toolbar.
If that doesn’t help, or you keep getting the error about a lock file, please get in touch with our Support team! They can help you make sure your data is safe, and figure out how to clean up that pesky lock error.