First of all, don't panic! There could be a number of reasons why this has happened. In most cases, there are ways to recover the lumens you have bought.
Are you using the same browser where you initially topped up?
Your wallet is stored on your browser where you did your top-up. If you are using another device or another browser on the same device, you will have to go back to the old device/browser to use the wallet again. Alternatively, you can also back up the wallet on your old device/browser, and then import it to the new browser/device. Read more about it here.
Have you recently deleted cookies on your browser?
Your SatoshiPay wallet is saved on your browser. If the cookies are removed, the wallet might also be removed. If you have made a back-up, you can import your wallet to your browser again. Click here for further instructions.
In case you have not backed up your wallet and you have lost your wallet on your browser, or you no longer have access to the device or browser where you did your initial top-up, please send the payment details of your recent top-up as well as your account ID to firstname.lastname@example.org. Our colleagues in the Customer Support Team would investigate the issue and provide you with the best solution within one working day.