Guys please help,im not a big tech guy so i want your help.

So today my brother was playing on my pc when the monitor suddenly turned off. I turned on the pc but the monitor said hdmi not detected.I thought there was a issue with my hdmi cable and i bought a new one and checked but still nothing. I thought maybe my monitor had a problem so I plugged it in my friends monitor which is perfectly fine but still same issue.I built my pc about a month ago, most of the parts like the motherboard, ram, psu are new the only used thing is the graphics card.Please guys help me this is my first pc and im scared as hell. Im gonna write the specs down if some of you think it might be an company issue

Note: the pc itself is turning on but the mouse keyboard stays off and monitors says HDMI not found

Ryzan 5 5600x(new)

Gigabyte b450m ds3h(new)

Evga rtx 3060ti(used)

650w psu(new)

16gb ram(new)

Please help me out im really scared as this is my first pc build and has costed me my entire pocket.