If you purchased an app and you deleted it for some reason, you can get it installed again on your Windows Phone without paying for it again. If you remember its name, go to Marketplace on your phone and tap on the Search hardware button. Tap on the app from the list and tap Buy. You will get a message on the screen telling you that you have already purchased that app. Tap on the Install to download it again on your Windows Phone.
If you have forgotten its name and want to install it, go to Windowsphone.com on your computer and log in with your Microsoft account (upper-right corner). From the drop down menu (located in the upper right corner), select “Purchase History”. Select the app from the purchase history and click on “Reinstall”.