I switched to a branch, but can't switch back to master? I cleared the cache but it doesn't show any other branches?

I realised, i had just deleted the branch the site was using (jharwood/approachReview). When i restored the branch i was able to see all the other branches including master.

I bumped into exactly the same issue. Would it make sense to have some option to default to the main/master branch of the repo? It looks like there's no other alternative but to restore the branch, switch the plugin to another branch and then remove it again. Branch removal is quite a common practice, therefore I'd love to have something like this (or any other solution). Thanks!
You can use WP-CLI to change the branch setting of the particular repo in the github_updater option.
The actual solution is to switch back to the default branch prior to deleting the branch you are on.
The problem with setting a default is getting a 404 response could simply be an authentication issue.
Ok, understood, the WP-CLI solution is good, so thank you!