[SOLVED] Desktop starts to turn on, then red CPU light and turns off again. Cycles repeatedly ?

[crowsaic]

I have a home-built PC desktop. It has been working fine for a number of years. Out of the blue when I tried to turn it on this morning, the blue fan light turns on, then after about a second, a red CPU light appears on the motherboard and it turns off again. It continues trying to turn itself on and shuts off after a second or two, repeating endlessly.

I've opened it up and tried reseating everything (except the CPU which is a huge pain to get to). Any ideas how to fix it?

Specs are below:

Case
NZXT S340 Glossy White Mid-Tower Case(CA-S340W-W1)
Wireless Adapter
Rosewill 300 Mbps 802. 11 b/g/n Wireless Adapter (RNX-N250PCe)
Motherboard
ASUS Z97-A ATX DDR3 2600 LGA 1150 Motherboard (Z97-A)
Hard Drive
WD Blue 1 TB 3.5 Inch 7200 RPM Sata 6Gb/s 64 MB Cache Desktop Hard Drive (WD10EZEX)
Crucial MX100 512 GB 2.5" SSD
RAM
Corsair Vengeance 16GB 2x8GB DDR3 16000 MHz PC3 12800 Desktop Memory (CMZ16GX3M2A1600C10)
GPU
EVGA GeForce GTX 970 4GB SC GAMING ACX 2.0 Graphics Card (04G-P4-2974-KR)
CPU Cooler
Zalman 135mm CPU Cooler Blue LED (CNPS9900MAX-B)
CPU
Intel Core i7-4790K Processor 8M Cache up to 4.40 GHz (BX80646I74790K)
PSU
EVGA SuperNova 850 B2 (850W)

Video of the issue:

 

[crowsaic]

I reseated all the RAM again and now it works.