Ryzen 5 3500( from prebuilt)What are the specs for this rig (make/model of all installed components)? Details matter, especially for the specific make/model of your power supply and video card.
Are you overclocking anything?
Was originally a prebuilt. HP Pavillion desktop. Bought beginning of 2020. Transferred everything over to my own case. Except mb, power supply about 6 months ago. No problems what so ever. Been playing vanguard since it came out like 2 weeks ago and then this happenedHow long have you been gaming on this PC?
Been playing Vanguard on it all along?
Try this. Disconnect your power cable from the outlet.
With the power cable disconnected, press the power button (discharges residual power in the system).
Reconnect the power cable and try powering on.
This sort of "resets" your PSU. If the system works afterward, the PSU may need to be replaced.