1.The easiest thing to do is to try re-installing the app. Delete the app and then re-download it and try to sign in.
2 If that doesn't work, then you will need to submit your issue to them for help on their contact support page . When you are ready, open that link.
3 Under Description of the issue, let the Facebook team know that you are having an issue with your Instagram account and that you require assistance.
4 If you can take a screenshot of the issue, do so and on this same screen click "Choose Files to include the screenshot.
5 Once you are done, go ahead and submit the issue to them and they should get back to you in 24-72 hours