Here are the options you should follow immediately:
Check your registered email inbox. If your mail ID has been changed, you will surely receive an intimation mail from Instagram. You can follow the mail content, click on Go back to this change and get your account back from there.
If you are still not able to get your account back, you have to report the issue to Instagram manually.
Here is what you should do if you are an Android user:
Start by tapping on the screen assistance to access right at login.
After it, you have to enter your email, Instagram user name or mobile number. Then tap Next
Click on Need more help? and follow the on-screen instructions.
Here is what you should do if you are an iOS user:
Click on Forgot your password? on the login page.
Click Need more help? in Send a login link option.
Follow the on-screen instructions.
What happens once you follow these steps?
Instagram sends you a mail that asks you to prove your identity. Here are the two things, Instagram may ask you to do:
Your one picture where you are holding the security code which is sent to you via your email. Make sure, it is handwritten.
The email and mobile number you have been using to access the application along with the phone's model number.
After completing it, Instagram will send you a mail with further instructions.