Black screen with blinking underscore

Hi there so I bought a used PC and Optiplex 790 and i upgraded the PSU and GPU for something better, however ever since I bought the computer i had always the issue of when booting and black screen with blinking underscore. I can boot into windows os after a 3min or so, sometimes i cant but i just retry by turning off/on the pc.I tried by upgrading my HDD to an SSD but it kept happening, so i tried installing a new fresh version of windows 10 pro on my computer, but no luck, so my last resort was updating the BIOS, which I just did but it keeps happening, however after a new windows and updating the Bios, the booting has become faster with the black screen with underscore just staying les than a minute.(Note: the hdd has been formatted and the SSD has the OS install and used as main drive. I also have the BIOS boot order to boot first the SSD). It doesn't bother my and I'm starting to think is either something on BIOS or a component like the CPU,MotherBoard,Or even the Ram. Just want to know if this is normalor if theres a way to fix this, im thinking in upgradding the componets but im sort of new in building PC, btw until recently i had no other experience with PC other than a vague 7 year old memories and school PC experience, ever since i bought the computer i increase my knowledge on technology, therefore now that i stand upon this i ask for someone to help me,either to tell me if this is normal or to guide me so that i can fix it.