Thank you for contacting the Intel Communities.
I performed some tests with the hardware below.
Intel® Core™ i5-6400 Processor
Motherboard: Asus B15M-C
RAM: KVR21N15D8/8 // 8GB 2Rx8 1G x 64-Bit PC4-2133 // CL15 288-Pin DIMM
PSU: Corsair HX1000i
1 core active, booted fine
2 cores active, worked fine
3 cores active, no issues
4 Cores active, booted without a problem
If you are performing OC (overclocking) the recommendation would be to load the defaults in BIOS and test the system without OC, if the problem is fixed, test the system with lower OC profiles.
Right know a GPU of that level is not available but the issue is no boot so that should not affect.