More information would definitely lead to clarity and better advice here.

Which item was it?  Was it the item you sold on March 16th for over £600?

Did you use the Ebay Simple Delivery label?  Can you provide the tracking number, please?

On a Simple Delivery label, that would begin 'YA'.

Ebay won't have opened an 'item not received' case.  It is up to the buyer to do that.

If you used Simple Delivery, once a case is opened, you, the seller, don't have to deal with it at all.

Ebay send an email saying the buyer has opened a case but they are handling it.  If there is a genuine case, they refund the buyer and you keep the payment.

Clearly, something has gone wrong here.

If Ebay have refunded the buyer from your funds, and you have proof of delivery, you can appeal.