You don't give us a lot to go on, and I can think of a dozen reasons how that might have happened. Hatti's suggestion that the duration is too short is one of them.
You might have accidentally deleted the title. If the project is still open, you can use Undo until it reappears, then copy it and Redo all your changes before pasting it back where it belongs.
If you've already saved and closed PD, you may find an intact title in one of the temporary project files if you have Autosave turned on. Check the folder specified under Preferences (gear icon) Project.
Is the title still on your timeline, but just isn't being displayed? Make sure that the "Display Title Track" checkbox is active:
There are other possibilities too, so please post back and let us know if any of these helped. If not, please tell us more about what you might have been doing when it disappeared, whether it's still on the timeline or not, and the steps you've tried to get it back.
Worst case is that you'd just have to recreate it.
YouTube/optodata
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