New install - computer will not boot into bios

I recently built a new computer, using a Z170-a motherboard, Intel cpu and gtx 970 graphics card. I double and triple checked all the coections before power up. However, when I powered up the machine, it would not boot into bios. I tried several tips off the inteet, discoecting and re-coecting all components (and powering down each time). However I still get the same result - no boot up. The DDR4 ram chips I used were 2 x 8gb ram (total 16gb), plugged into the correct sockets. I just can't understand why I can't boot up the computer. I've built several PCs in the past and have had no problems. Any ideas?