2400G installed, now lost M.2 boot

there was a post earlier about another model asrock board same issue

cured with the later beta bios eg...... 4.63a

Thanks.
I did not realise there was such a BIOS for this MB X370 fatality gaming ITX I will have a look, but first do a BIOS  reset.
First thing I did when I powered down to replace the CPU with the 2400G was CMOS jumper and CMOS batt, after disconnecting powerbrick and holding start button in for 30 seconds.
No harm tring again though

insitu boot repair or m.2 hooked to another PC?
 and hopefully  windows repair rather than reinstall....although at a pinch I could reinstall and drag what I want back from win-old files... done that before

I don`t know why I assumed a Windows repair is needed, it is not even getting near windows, it is the boot drive that is missing, not the boot media.
I think the issue is with BIOS and something  in UEFI or Legacy mode settings  that I have not worked out yet.
I know there are settings in these that must be altered for the installation of windows installlation files and that is what has me thinking that because I cleared the BIOS prior to installing the 2400G i somehow lost these settings.
Probie, not sure why you did not have the GPU part working,are you definite you have the up to date BIOS for Raven ridge?
You do not mention what motherboard you have, or what your full build is.
It can help other help you if you list the spec in your signature box.
Lastly, do not let this put you off AMD and send you back to Intel without at least overcoming these initial issues and giving AMD a fair crack of the whip!

Thanks for input:
I forgot to mention, I did a full CMOS clear as soon as the board was powered down for the CPU-APU swap, kettle lead out, start button held down for count of 40, CMOS clear jumper switched to clear pins, CMOS battery disconnectd via 2 pin connector block on motherboard. It is not practical to lift out the battery on this MB because it is the type that is in a plastic jacket strapped to the back of the I_O ports.
These 2 items were left in the "clear" position for about 40 minutes and the BIOS was back to default when it did boot into BIOS.
Upon the first failure to boot from the M.2 the CMOS clear was undertaken once more.
I think the problem lies in a BIOS settings somewhere, Legacy etc, and I will be having another look at the options there.

Point of interest.
The M.2  originally had Windows 10-64 installed from  an ISO file whilst on an ASrock  AB350 fatality gaming ITX, when I got an X370 fatality gaming ITX I just lifted the drives ram CPU etc from the AB350  with BIOS 3.10  and an R3-1200, and straight to the X370.
The x370 booted first try  and ha been running fine with no issues.
The X370 at that stage had BIOS 3.0
I updated the BIOS on the x370 to 3.6 Bridge, then to latest 4.4, then undertook the CPU-APU swap.
I doubt the non-boot is a Windows issue because Windows is not even coming into play, I am convinced it is a BIOS setting issue and as I  said I am going to look again.

A bit long winded ,but I wanted to give the full picture otherwise it is unfair on those trying to help.