I was sent a money transfer from the UAE. Unfortunately, I gave details of my closed account to the sender. The receiving UK bank could not credit the money to an account so returned the amount to the sender. After 2 weeks the money has not been received by the sender and HSBC in Dubai keeps on saying they are not able to trace the money. What can I do to find what has happened to the money?
This is not your problem.
More to the point you won't be able to find out details. It's up to the person who sent you the money to find out what happened to it. All you know is that you haven't received it in your account (because it was closed).
No bank is going to give you details about a money transfer sent by someone else, so you can't help the person who sent it. They need to talk to their bank to find out what happened to it. The most you can do is get as much detail as you can from your bank (which may include where they received the attempted transfer from and where they sent it back) and pass those on to the sender to help them.