Hello,
If you used the beta version before I suppose you have XStore core plugin 1.1 version and it can be the reason of the problem. Connect to your site using FTP and rename the et-core plugin folder. Check if you can login to Dashboard after that and if yes then remove this version of plugin and install the one that comes bundled with the 4.28.1. If plugin is not the reason of the problem then provide us with temporary FTP and Dashboard access to your site.
Also, could you tell me why did you decide to change the version 5.0 back to 4.28.1, what issues did you face?
Regards