"Dear Cyber Guardian, someone logged into my Instagram and changed the email. My friends are getting weird money messages from 'me' and I feel sick about it. What do I do?"
— Locked Out in Year 9
First: this is not your fault, and it is fixable. Go straight to Instagram's 'Need more help?' / hacked account. Because the email was changed, they will send a recovery code to your original address or ask you to verify.
While you wait, post from another account or message a group chat telling friends the messages are a scam and not to click anything. That stops the damage spreading or other accounts getting hacked.
Once you are back in, change the password to a long and strong password you use nowhere else, turn on two-factor authentication, and check Settings → Security → Login activity to kick out unknown devices. Finally, change the password on any other account that used the same one.