Eunice here from Shopify, thanks for posting, and welcome to the Community!
This can sometimes happen when we haven't cleared our browser's cache and cookies in a while. Our browsers often hold on to old information causing issues like this to arise. To fix this please clear cache and cookies, restart your browser, then try again. If you're not sure how to clear cache and cookies this guide will show you how.
If you're still not able to log into your store after clearing your browser's cache and cookies, please visit the Help Center to create a support request. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. This video clip shows you how to access the chat and email option. If you would prefer to speak with someone over the phone, please log in to your store when prompted to view the callback option.
I hope this helps, if you have any issues contacting Shopify Support please let me know here. Cheers!