PC turned off suddenly. Now whenever I turn it on, this happens. What should I do?

It's a new PC I built two days ago. It was working fine, but then suddenly it switched off, and whenever I try to switch it on, the RGB just turns on for a moment and it switches off. I don't have an extra PSU to try. What's causing this problem and where do I start to fix it? Is it the case power button, PSU, RAM or CMOS battery or what?