You seem to want a shotgun fix rather than methodical troubleshooting & diagnosis, but you haven't listed all of your specs, which is problematic. I will try to help, but with so little info I can't be very precise. I have built a lot of Ryzen systems though, so I might at least be able to point you in the right direction.
#1 reason that I have seen Ryzen be unstable is untuned memory.
#2 reason is too high wattage PSU causing severe voltage fluctuations.
#3 reason is too high core voltage. (Drop it to improve stability.)
#3b reason is too high LLC or inverse LLC (some motherboards) which causes #3.
Many motherboards have bad RAM profiles, especially for Corsair memory. They will pull incorrect settings which you must override. G.Skill and Kingston profiles are pretty good, so many of those drop in okay. Then you just enable XMP and set the right speeds and voltages and off you go. The latest beta BIOS's have improved RAM compatibility a lot, so grab the latest betas if RAM is your trouble causer.
For testing you will want to set your RAM voltage to 1.4 or 1.45v. A lot of people with Ryzen will buy 800-1000w PSUs of bronze or lower quality. Power supplies run best in the 20-75% load range. Down under 5% load, like when just starting the boot process, some lower quality PSUs have RAM voltage fall well out of spec. I have seen 0.06v droop. (1.2v -> 1.14v) which is obviously completely atrocious. The only solution to such enormous droop is to crank that voltage up. If it's rated 1.35v, set it at 1.40 or 1.45v. That ought to do it. Hopefully it doesn't do the inverse of a droop and fry it. Samsung B-Dies can take like 1.7-1.8v before death, so there is some wiggle room here depending on your RAM, but if your PSU or mobo VRMs regulate poorly, expect gradual RAM degradation from voltage variations, and eventual death. And buy a better PSU next time. I run a 550w Titanium one. They were cheap - nobody wanted those for mining. It was like $120, and the voltages don't even change in software. They just stay the same. Really tight regulation.
Okay, you have 3000mhz RAM. That's less than ideal. Ryzen works best with 3200mhz RAM. (or 3600mhz for the 3000 series... soon Ryzen CPUs with 2000mhz internal will exist, so then 4000mhz will be ideal for them). I would try to hit 3200mhz, even if you have to loosen your timings 7%. Just go in and check all the Auto settings, bump them up 7% with a calculator, rounding up. Then try 3200mhz and see if it POSTs.
If you're on 1st gen Ryzen, set voltage to 1.4-1.45v. For 2nd gen, 1.3-1.35v or less. For 3rd gen, 1.2-1.25v or less. Start at stock speeds, turbo and C1E power saving off. After you get a clean Windows install you'll bump up your stock speed slowly, then run OCCT and see if your system crashes/freezes. I have always found that Ryzen systems die if you overvolt them. (Which happens on Auto settings.) By dropping voltage a bunch, I could unlock several hundred mhz extra. It'd look something like this:
Ryzen 2700x 8 core
1.150v: 3.7ghz limit
1.175v: 3.8ghz limit
1.200v: 3.9ghz limit
1.225v: 3.9ghz limit
1.250v: 4.0ghz limit
1.275v, 4.0ghz limit
1.300v, 4.0ghz limit
1.325v, 4.0ghz limit
1.325v, 4.0ghz limit
1.350v, 4.1ghz limit
1.375v, 3.8ghz limit
1.400v, 3.7ghz limit
1.425v, 3.6ghz limit
Ryzen is not over-voltage tolerant, so to get a really stable system you need to do the tuning before you dive into using it (as you may corrupt your OS), and select a high quality power supply before you start a build. Otherwise - headaches ensue.
Hope it helps. P.S. Best to be methodical if you want to know what actually fixed it in your case. But if you do all that, there's a good chance it at least POSTs and you can install Windows and start tuning from there.
A list of all your parts would certainly help with being more specific.
