did he change the account phone number and email? do you still have access to your email and the phone number is still the same? if the hacker did not change any of those information you can reset the account password and get it back.
Usualy the first thing a hacker does is change email.
Look for emails from instagram saying your password/email changed and click “Revert changes”.
If that doesn’t work,you have to report your account as hacked.
Follow the IG steps from withing the app,i think you can only report it if its US base account.
There is no real way to contact IG