You have 30 days from the actual or estimated delivery date to start a claim with ebay.
If the item has not arrived select "Report an item you didn't receive" on here:
https://www.ebay.co.uk/help/buying/returns-items-not-received-refunds-buyers/get-help-item-hasnt-arr...
and escalate it 3 complete working days later.
If the item arrived but is not as described start a return on here:
https://www.ebay.co.uk/help/buying/returns-refunds/return-item-refund?id=4041
select "Doesn't match description" and escalate after 3 complete working days.
If it's more than 30 days it's too late to do anything via ebay but contact your payment provider and ask about a refund or chargeback.
