As you are getting the looking for the OS symbol (blinking question mark) the drive is not recognized. Is this the original drive or did it get swapped out?
If it’s what the system had then it’s possible the socket got damaged or the contacts are not making a good connection.
If this is a different drive then it’s either just a simple correction as the expected drive is not present (or the setting got lost). Restart your system pressing the Option key to get to the Startup Drive manager, then select the boot drive.
If that fails then you have a newer file system on the drive than what the system can support as the systems firmware was not updated to support APFS from the older HFS+ file system. Using Bootable OS installer with Mojave or newer, no higher than macOS Big Sur (macOS 11). So you can upgrade the systems firmware and refresh the OS.
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