Have you made a purchase but have yet to receive a confirmation email or can't see the tickets in "My tickets"?
This could be due to your order becoming expired (due to either a technical failure or the transaction taking too long). Below are the instructions to confirm whether this is the case. However, in most cases, you will need to place a new order for the tickets.
Sometimes this error appears due to the long waiting time on the payment page or an issue with the bank processing the payment. Your bank may still reserve the amount required for the order. This amount is automatically released back to your account, but it can take 3-5 working days to appear but this depends on the bank your using.
You can check if your order has gone through by logging in and clicking on your name in the top right, then Profile -> My Data -> Orders. If the status of your order says "Expired" it means that the transaction did not complete and you should try again.
TL;DR: If you haven't received an email or can't see the order in my Tickets, the process failed - your money will be automatically returned to you should try again!