No signal to monitor from (sort of) booted pc

I recently built my first pc using parts I bought online. After pressing the on button for the first time I was delighted to see the case fans, cpu fan, graphics card fan and the hard drive spinning. However when I plugged the cable into my monitor nothing happened. My monitor is working fine as my other pc confirms and I've tried multiple different cables and monitors. The monitor still has no signal, any help would be much appreciated:)