big sur 2 installed now computer wii not start up

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.
big sur 2 installed now computer wii not start up
 
 
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