Yesterday, Microsoft released Edge Dev build 78.0.268.1 with some minor new features such as the ability to reorder Collections with the keyboard. Apparently though, there was an issue that caused it to not work in S mode (or Windows 10 S), the version of Windows 10 that only runs verified apps. There was a problem with the digital signature, so some users saw an "unverified publisher" message when trying to use it.
That issue is fixed in today"s update, which is build 78.0.268.3. That"s the only change in the build, so if you"re not on Windows 10 in S mode, you don"t have to rush to update. If you are on S mode and you"re unable to open the browser, it should update in the background at some point, and you"ll be able to use it again.
The Dev branch is updated weekly, so these types of issues aren"t all that uncommon. If you want something that"s more stable and less likely to leave your browser in an unusable state from time to time, you can always try out the Beta branch, which is updated every six weeks.