If you no longer have access to your email account, first check yourself whether your email account has been hacked.
If you still have access to your Instagram account, follow the instructions for case 1. Additionally, change the email address you have on file with Instagram in the Instagram settings under “Account” -> “Personal Information”. This way, your Instagram account is secured for the time being. However, it is especially important that you also secure your email account again. Because if the hacker still has access to your email address, they can undo the changes you made in the Instagram settings. To do this, all they have to do is click a link that Instagram sends to your hacked email address. As a result, they can set a new Instagram email address and you will no longer be able to log in to Instagram.
If that happens, follow in the instructions for case 1 above, where you no longer have access to your Instagram account.