At the moment some of your external modules (that are installed and turned on) does not want to work with 7.3.
ONLY IF YOU HAVE COMPLETE AND SECURE BACKUP WITH ALL YOUR FILES IN MODULES folder you can start deleting the modules folders for all external modules (starting with the most likely bad players, probably you know which modules don't play nicely lately).
After each deletion go in your modules page and refresh. If again you see blank continue until you remove the bad module and you can access your Modules page. Then you can copy back all deleted modules in the folder except the bad one.
Then you we can proceed with troubleshooting, updating, migration, etc.
Don't install, reinstall and make quick and stupid moves if you're not sure in 110% in your backups! This can be fixed very easy.
If you install fresh copy and paste db you will end up with a shop copy with no media files (for products, theme, etc). I doubt you wish to add them all one by one after that if this is not the very last resort.
Regards!