My PC Won't Boot - Any Suggestions?

I'm relatively new to PC gaming and recently bought a new system. Everything was working great until yesterday when I turned it on, and I noticed that there's no display and my peripherals aren't getting any power. I've already checked all the connections, including both RAM sticks and the slots, and I've taken out and reconnected my GPU. I'm running out of ideas on what to try next—could this be a power supply issue? Would the lights on my GPU come on if that were the case? The fans have RGB lights and are spinning, so I'm a bit confused. Any further suggestions would be greatly appreciated!