If any of your in-app purchases have disappeared from your game, perhaps after a reinstallation of the game, these in-game purchases can be restored via the game's Restore Purchases button. This button can be found at the bottom of the in-game store. Simply click the button, and the app will attempt to contact Apple and redownload any previous purchases. Once Apple's server confirms the previous purchase, the download should begin and the content should become available.
Assuming you are signed into your device under the correct AppleID, under no circumstances should you be charged a second time for any in-game purchases.
If you experience this issue on a device other than the one where the original purchases were made, please check here.