Hipstamatic

Go to the iTunes Store, log into your account, and click the Purchases link under the Quick Links. From there you should be able to re-download that app. You can also re-download content using an iOS device.

For full instructions, see:

http://support.apple.com/kb/ht2519

Regards.

Any chance that the app was purchased through a different iTunes Store account?

If the app is in iTunes on your computer, select the app and Get Info on it. That will tell you what account the app was purchased through. If it's not in iTunes, I don't know of any way to find the account and without knowing that there's no way to recover the app without purchasing it again.

Regards.