Windows 2018 security and/or cumulative patch does not like Athlon processors.
I have AMD Athlon 64 X2 5600+ and on same Machine using Windows 7 32bit and then Windows 10 64bit v.1607 was unable to boot or get into safe mode.
Found this post on two websites but have not tried it yet. looks promising.
In the first link, he posts he found a solution:
Finally found a solution to remove the update package using DISM. On startup, press F8 and select Repair Your Computer. From there, open a command prompt window. Check that the Windows drive is mapped by running
Run the command
dism /image:d:\ /remove-package /packagename:Package_for_RollupFix~31bf3856ad364e35~amd64~~7601.24002.1.4 /norestart
NOTE you will need to find the name of the patch for your OS (Windows and 32 or 64 bit) and replace that in the above run command.