First, take out the new GPU. Do a CMOS rest of the mobo - check the mobo manual on how to do this. The purpose of doing the CMOS reset is to be sure you have the mobo video enabled. It may have gotten disabled when you had the GPU installed. Hook up the monitor to one of the mobo's video out ports. See if you can boot up to the bios and/or windows. If you can get this far, you know that the new PSU is OK.
Now go back and reinstall the GPU, and switch the video cable to the GPU. Hopefully, you'll get a video signal and then install the GPU drivers. If not, maybe a bad GPU that you need to RMA.