If you're running OS 7.1, it would be a 9810 you have and not a 9800, but no big deal.

First of all, on your PC browser, type
www.blackberry.com/desktop to get Desktop Software back. Next, on your phone, open App World, go to My World, press the menu key and select Refresh List. Once your list has been refreshed, you should then see the missing app under the Uninstalled tab. Reinstall it then remove your battery while your phone is on then replace it. Once your phone has rebooted, plug it in to your PC and open your newly installed Desktop Software. Do a data backup by clicking Device at the top of the software then Restore.
Let us know if that helps you.
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My Z10 was furnished by BlackBerry but all opinions are my own.