Using onboard iGPU to boot the PC, then check your MB BIOS version, how to http://www.howtogeek.com/196916/how-to-check-your-bios-version-and-update-it/ If the MB does not have the newer one, 0902, and update the BIOS. Then try use the hd 4850 again.
May also test the HD 4850 in other PC to make sure it works or not.
If it works in other PC, and you also update the BIOS to 0902 version, but the PC still has problem with the HD 4850. And the MB (pciex16 slot) may have problem.