This is the kind of issue you’ll likely need official support from Axure. Send an email to email@example.com and if possible, include your .rp or .rpprj file (or link to it if large.) They are responsive and have been quite helpful for us with similar issues.
In the meantime, if you haven’t already, try to sign in to Axure Cloud from your browser. If that works, then you know you have a good internet connection and account. Then try signing in from Axure RP (upper right) --sign out and then in if need be. It could be likely that the updated Axure RP did not maintain your sign-in. If that doesn’t work, reboot your system and try again. If all else fails and you haven’t received a reply from Axure support, you can download a previous version of Axure RP and that should work if it worked before.