@jimLee Useless response. I found the issue, in case you're curious.
It's with the new boards package for the Nano mbed boards. Went back to the IDE, deleted the programs file and reinstalled the previous version of Arduino Mbed OS Boards (1.3.1). Everything works again.
2.0.0 needs a fix. And not being able to choose which version of Arduino Mbed OS Boards is being used in the Web Editor is something people need to watch out for.