If you use Windows, you may have encountered a problem following March's security update, in that Burp Proxy listeners running on the loopback interface stopped working. This was caused by Microsoft changing the "localhost" entry in the Windows hosts file from:
Manually reverting to the old entry fixes the problem for a while, but Windows will silently update to the new entry periodically. (Note that if you are running Windows Defender, you may need to dismiss some alerts in order to modify your hosts file.)
The latest versions of Burp (both free and Pro editions) have been updated to work with the new hosts entry. If you were having problems, please download the latest release and things should start working again.