After the testing period has ended for a TestFlight Beta, the build status changes to Expired

Your TestFlight app users should not be able to launch the app 90 days after you release it due to provisioning limitations.

If you want your test versions to last beyond 90 days, you can use good old fashioned ADHOC builds (but you'll be limited to 100 users versus the 10,000 you can use via TestFlight).

If you have a tag/access to the code of the build that has expired, a simple bump of the version number and a reissue to iTunes Connect makes the build immediately available for another 60 days.

I had this problem this morning but needed to keep the groups and try to get a build out as cleanly as possible and so that worked a treat.