@Danyal Sheikh I understand this issue could be frustrating. Just to highlight, since you’re able to access it from Azure but not from SendGrid (dashboard), and also the issue is not specific to the Azure tag where it’s posted. Based on my research I want to help best here, I’m not sure if you have tried these already, but they’re worth a try.
You have stated Username and Password is not available from “Account Details” (SendGrid page), kindly double-check if you see the same registered account on that page (screenshots below)
Check your registered email ID for the account details /username and then if feasible reset the password (as required – which is the only option available from Azure Portal to manage SendGrid account and that the comprehensive SendGrid management is done from SendGrid Portal/dashboard).
I’m not sure if you’re the only admin on the Azure and SendGrid, if there are other co-owners/co-admins, kindly check with them to make sure they haven't changed anything.