I'm building a new media centre PC; it's all together, but I can't work out why it won't reach the POST screen.

When I hit the power button, the CPU and case fans spin, and the power LED comes on, but I get no display output. I'm using on-board graphics, outputting through the motherboard's ports. I've tested it with two separate monitors, and both DisplayPort and HDMI outputs, to no avail. I've tried re-seating the RAM, but I'm not 100% sure it's in right - the little clips just don't go in, no matter how hard I press. If anyone has any other suggestions or advice, I'd love to hear it.

My parts list is:

• ASRock B660M-ITX/ac motherboard
• Intel Core i3-12100 CPU
• Noctua NH-L9i-17xx CPU Cooler
• Corsair Vengeance LPX 16 GB DDR 4 RAM
• Western Digital Blue SN550 500GB SSD
• Corsair RMx 550W PSU
• Thermaltake Core V1 Mini Case