I bought an imac 27" fall of 2019 I am just setting it up now . I have been suucesful up to the point where it asked me to install 'big sur' It shut down and will

imac 27" I am just setting it up now. I got as far as the part where it asked me to install big sur when I came back, the computer had shut down.. It will not restart, and only momentarily displays the apple logo(and nothing else) fore about 4 seconds and then the screen goes blank again... I pressed the power button for 10-15 seconds to force it to turn off and then pressed the power button to start it, but have the same problem