As @offlinehelpers said, you’ll need to contact Customer Support.
Unfortunately, it’s possible that you won’t get your funds back. When the seller delivers the order, you have 3 days to request revision or mark it as complete; as you have found out, if you do nothing for 3 days, the order will be automatically marked as complete. The seller doesn’t get funds immediately, they get it 14 days after the order was marked as complete, and during that time, you can request a refund. Since you have waited for 3 weeks, that option might be gone.
It’s all explained in Fiverr’s Terms of Service (it’s not legal speak, the ToS is easy to understand and explains how the site works both for buyers and sellers, and what to do in certain situations).
Other than contacting Customer Support, you can also leave a review (you have 30 days to leave a review after the order was marked as complete) and tell your story.