PC parts work individually, but don't work when assembled togheter

About a month ago my PC stopped showing any display. The CPU debug light also turns on whenever I boot up my PC. I took it to a repair shop and they tried diagnosing it. Every part passed the test and worked on its own, but the PC as a whole just refused to work when assembled. I have an AMD Ryzen 7 5800X CPU, MSI B550 GEN3 motherboard and a Radeon RX 7800 XT GPU. I genuenly have no clue what I should do at this point or how I could get it fixed, and I really need help.