When I check my connection with https://projectstream.google.com/speedtest it says my connection is great...and I was able to play with great perfromace yesterday, but today, my connection is still great but stadia will not allow a connection?
Try again later and see if the issue has resolved itself (could be stuff being done on the Google infrastructure that has caused this issue for you). If still no luck, try a restart of home router and see if that makes any improvement. If not, reach out to support and raise the issue with them, explain that it was working successfully before, and that you have tried restarting your own side of things, Google stream reports good speeds etc. and see if support can investigate further.