Do you have a recent backup of your phone on iTunes? You can restore to an earlier back up and it will be there. You'll lose some current data, but it may be worth it.
If you've synced your device to iTunes on your computer and allowed for the transfer of apps and app purchases, iTunes should still have the app saved.
If all else fails, search the app in the App Store from time to time. Sometimes a developer will remove an app, then later introduce a new version with the same name. I've seen that with a couple of my apps.