A cloud-based identity and access management service for securing user authentication and resource access
I was able to resolve the issue. Using a Chrome incognito window with all browser data deleted, I went to https://signup.azure.com and used my old email address (the same one that was blocked when I went to portal.azure.com). It recognized that I had an account and free trial credits ("You previously signed up for an Azure free account and still have free services available to use. Continue using these services by upgrading your account, and pay only for what you use above the free amounts"), so I had to "upgrade" to a pay-as-you-go account (although I was able to stay with a "Basic" account with no charges). It doesn't make any sense to me why Azure couldn't tell me that on the portal.azure.com screen or why this would even be necessary since I seem to be on the same terms as before.