Are you able to share with me a screenshot of what exactly you see when you attempt to login?
The first thing I'd to rule out here is a local issue with either your device or browser - can you please go through each of these steps one by one and let me know if this works:
Clear your cache and cookies on your browser. After this, completely reboot the browser.
Try on another web browser or an incognito window.
When using incognito, if you are still experiencing a display issue, confirm you have disabled browser extensions as these can change how the pages look.
Try another device or on the Shopify Mobile app.
Try another internet connection (example, switch to a cellular network on a phone rather than wifi.)
Ensure there are no pop ups being blocked in the web browser.
Ensure you do not have any firewalls or pop up blockers enabled on the device
If the problem still persists after completing these steps, then the next step will be to reach out to our live support team. Our live support via phone, chat or email can be reached via oursupport portal here. Simply type your query into the "ask about a topic" bar, then hit continue to support to access.
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