I couldn't find anything this morning regarding this beta update problem and tried to fix it myself Big Mistake: I installed the latest big sur beta last night. Now my imac will not boot up. It started with shutting down about every 20 seconds and then rebooting itself. I booted up into recovery and tried to reinstall the operating system, only choice was big sur. Same problem with it rebooting every 20 seconds. Rebooted again into recovery and this time chose reinstall from time machine. Chose a date from before this last beta update. Now I hear the start up sound, then the apple logo then the slider bar goes half way and a black screen appears. I cannot do anything. I tried to boot up into recovery, nothing, tried completely turning off the computer and unplugging it, still nothing. I think maybe my time machine backup was missing the operating system? Dummy me, it is my main computer. 

Mac will not start after installing big sur beta 2. Tried turning off and on again. Tried unplugging and then turning back on. Get the normal start up sound, then the sliding bar moves a little, then black screen. What can I do? Dummy me, it is my main computer.