If you are able to login, the page is loading, but not all of your notes are visible (just partially synced), and you have waited at least 15 minutes (if you have a lot of notes, and the internet connection is fine), see here.
You were able to login, but the title says "Loading..." without a change, make sure your firewall or VPN settings (like "ZSCALER") allow access for "https://api.apple-cloudkit.com" and "https://app.noteplan.co". You might need to bypass the SSL inspection for these URLs