What's going on with my pc

I have a Dell Optiplex 9020 PC. At first, it worked fine. Then I shut it down. The next day, when I turned it on, a 'no bootable device found' warning appeared. Then I wanted to enter the BIOS menu because there might be an incorrect legacy or UEFI configuration. But I couldn't enter and instead got the error 'unable to run setup. Halting the system'. Then I opened the PC and replaced the BIOS battery and reinstalled the RAM. Also, the SATA cables have been replaced. But the result is still the same. Why is this happening?