As far as I know there hasn't been a user-wide hack with both bitcoin.com and bittrex, so I'm going to assume your funds got lost through malware or the typical phishing.
Unfortunately, there's really nothing you can do right now. If you lost a huge sum of money, like huge huge where it's worth hiring lawyers and stuff, then do so. Your hacker might do something stupid like move the funds to a centralized exchange.
Did you use 2fa?
Did you have friends that make use of your device you use to access the exchanges often?
Are you making use of the internet and not care about the safety of your device like having antivirus and also visiting only legit sites?
Unfortunately, there's really nothing you can do right now. If you lost a huge sum of money, like huge huge where it's worth hiring lawyers and stuff, then do so. Your hacker might do something stupid like move the funds to a centralized exchange.
Lost of bitcoin on two exchanges at ones, this will likely be an hack like you first said or maybe someone he knows stole it by accessing his device used for the exchanges.