What do you see instead of the login? Is the screen completely blank or do you see some sort of error message? Or does nothing ever happen after you get the splash screen?

One possibility: We worked with one customer in France who was getting a "bad request" message in the login widget, instead of the normal UserName/Password field. Turns out that component was having an issue with his en-europe region setting.  Changing to en-fr worked fine, as was en-us.  

Another possibility: We've seen an issue on laptops with intel iris xe graphics cards where the login widget would be completely blank, or users would see the user/password fields but would not be able to type in.  Turned out to be an issue with Microsoft Technology on those graphics cards. The very latest update should have addressed that.  Try start menu, ProSeries 2020 -> Tools Repair Updates.  If you still have this issue, go into ProTax20.exe.config (using Notepad or Word) and change 'Default' to 'Standard' in the following line. Then save and restart ProSeries.

From

<add key="Intuit.Spc.Authorization.EmbeddedWebBrowserImplementation" value="Default" />

To

<add key="Intuit.Spc.Authorization.EmbeddedWebBrowserImplementation" value="Standard" />

If you're seeing something completely different, or none of the above is working.. can you email the following two logs to proseries_engineering@intuit.com

[installation directory]\32bit\logs\proseries.txt

C:\Users\{YOUR USER ID}\AppData\Local\Intuit\Common\Authorization\v3\Logs\Intuit\IntuitAuthLog.txt

Note, the second one is in a hidden directory. If you're not sure how to get there, type %LocalAppData% in the Start Menu. This *should* bring up File Explorer to C:\Users\{YOUR USER ID}\AppData\Local. Then you can browse from there.