hp 15-n025sl three beeps

Main : DA0U93MB6D0 REV : D

Hello i have a problem with this notebook. As i write in the title this noteebook presents three beeps and so no video. After reballing of gpu and then of the CPU AM5200IAJ44HM i solved this problem but i have now another kind of problem. When i turn on the pc i have video but it says no boot device please install an operating system on your hard disk. If i do the hard disk test it is ok and i have tried also to reinstall operating system with a bootable pen drive and dvd but i have no success. I tried also to rewrite bios and change hd but nothing changes. Can you help me?Thanks