Making sure build powers up

My computer build is nearly finished, it is missing the GPU and optical drive, it was working fine the fans were spinning and such all LED lights were on, no problems. I open it all up to do some cable management as it was a jungle in there, replug things, re-arrange things etc and now my pc wont turn on, every now and again when I click the power button a LED light on the mobo comes on for like half a second then goes off, no fans nothing.

I've unplugged everything again and I'm going to redo it on wednesday when my parts arrive, basicly what I'm asking is what should I specficly make sure are plugged in properly, what things should I double check and such.

I have left the power connector in as it was a bitch to plug in and out, but I assure you it is as far in as it can go (I learned from previous problems).

Does the PSU fan spin? Are you getting any beeps from the motherboard?

When i have this problem I reduce it to the basics. Only plug in the power(24pin and 4/8pin) to motherboard, heatsink fan, check ram is seated, power button plus speaker for case and the monitor cable. See if it boots to bios and heatsink fan spins with only that. If it doesn't then we can proceed with futher help.

make sure everything is plugged in, make sure the psu is switched on

I had this once too, but everything just wasnt plugged in correctly