Certainly nothing to worry unduly about
It will be a eary logo screen set in BIOS -
Initially Bios graphics may be 800x600 but you screen is set to a much higher value hence you only see part of the full logo screen. BIOS then adjusts the settings as it moves through POST.
I assume the machine is 4 /5 yrs old? and switched off at mains regularly? Its possible the CMOS battery is dying and the BIOS has reverted to default the screen may have been disabled by whoever set up the machine initialy (your manufacturer).