To my knowledge, ringtones are not backed to the cloud, as are most other purchases. They must be manually backed up. If not, they are lost and must be purchased again.
I have had this happen. What you can try, is call Apple and request to speak with the iTunes department. This was years ago, but they were able to restore my lost ringtones as a one time exception. I have no idea if they still offer this, but you can certainly ask. I lost a total of three ringtones and all three were restored. Good luck.
If you did not backup your ringtones to iTunes you're screwed. I made that mistake one time. I now have them backed up on multiple computers; just in case.