You can reach out directly to Uber to help locate your spend history via the Uber Help section in the app or to help.uber.com via desktop, as only their agents can look into your account and any activity on it.
Below are instructions on how you can view your credits in your Uber app.
Go to the homepage
Click on the upper right hand symbol
Click on wallet and your credits will be displayed here.
Click on the "Uber Cash" section (Please note apps can slightly vary per country, Payments may be called Wallet and Uber Cash may not exist or be the terminology in your region)