It's hard to help without more information. When and how did you pay for this item, and what was the reason for returning it?
Did you return it through eBay's returns procedures, and comply with their conditions and procedures? These are explained here:
Returning an item for a refund: https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
eBay's money back guarantee: https://pages.ebay.co.uk/ebay-money-back-guarantee/
If you can explain exactly what you have done so far it will be easier to suggest anything you may need to do to get your refund on track.
If it's all gone through correctly and eBay has confirmed your refund, it can still take some time to reach your account. Credit card payments are the slowest to refund, and can take up to a month.
