If the address that you sent the bitcoins to is correct, and if the transaction is actually confirmed in a block, then the ONLY people that can help you are Coinbase employees.
What has coinbase done with my btc,
Only Coinbase knows. It's impossible for anybody else to know what Coinbase has done.
is it lost forever?
That depends on what Coinbase has done, and how convincing you can be to get them to help you.
Thanks for any help or advice.
Good luck. If it was a large amount, you might want to contact a lawyer.