I've been running the iOS consumer beta for a few months largely without issue. Today it offered to install the latest update. The battery was highly charged and I had it sitting on a wireless charger and it failed in the middle of the update. Now it just cycles with the white apple logo showing and off in a 5 second loop. I got it to connect both to iTunes and imazing but neither could fix the problem, no doubt because they are trying to do ios11 and running into conflict with iOS12. Help!!!
Put it into DFU mode and use iTunes to restore it. You can restore it to either iOS 11 or iOS 12 beta 9.
Here are instructions on entering DFU mode: https://www.theiphonewiki.com/wiki/DFU_Mode (Since the instructions vary between devices and you did not mention which device you have, I won't copy and paste them all to this page)
When it's connected to iTunes you should be able to press Restore. If that doesn't work, try restoring a specific IPSW file. You can download files here: https://ipsw.me
You can use the IPSW by holding option (mac) or shift (windows) when pressing the "Update" button. It will prompt you to select the IPSW.