FIRST THING TO CHECK: if your components work outside the case, but don't work inside the case, then it could be improperly wired Power On button in the case. I've seen this with Fractal cases. Cheap Chinese manufacturing and the power button is miss-wired. Will not power on computer, and may ruin the mobo. Try taking every out of the case again to make sure all parts are still good.
SECOND THING TO CHECK: Make sure front panel headers are connected properly. One pin off, and it won't start.
Tertiary Troubleshooting:
1. Check all power connectors. I know you checked them. Double check. Make sure every plug is plugged firmly, including wall socket.
2. Check GPU is seated properly. Sometimes it's not all the way in, but looks like it's in.
3. Check SATA cables, make sure all connected firmly to Drives and to MOBO.
4. Check RAM is seated correctly, and in the correct slots.
5. Unplug everything that is unnecessary. Fan controller, other nonsense, get rid of it.
6. If everything checks out, then you may have shorted out the MOBO when you were installing into the case. It is very easy to do, a small static charge will do it. Especially if you're working on carpet.
7. You might also have shorted out the RAM modules, either one or both with static charge and/or rough handling. Dropping them can damage them.
8. If your components are good, then something is shorting out the PSU, some bad connector somewhere. Did you resleeve your cables? If yes, then check all the connectors to make sure all pins are in correct alignment.