I'm interested in investigating your case, but..
First of all you're victim of a crime, and if someone additionally blackmails you, two crimes. You should inform the police immediately.
Secondly describe the scenario a bit better please, did someone takeover your site and encrypted all its contents?
Thirdly is the RSA almost uncrackable . If there is no trace of the key somewhere on your site, it'll be impossible.
Don't you have backups?
My bid is only a rough estimation for 3 hours of work. If there's no trace of the key, a decryption is impossible.