Bitcoin transactions are irreversible. So unless the hackers/scammers send you back the bitcoin (may be under threats or for whatever reasons), there is no way you can get it back.

If you have been hacked, you should check your machines for malware and store your bitcoin in a better way (like in an offline wallet).
If you have been scammed, you should learn to use escrows and be careful with "too good to be true" trades.
If you have lost your own wallet file, you should remind yourself to make backups next time.