With the new card installed AND the supplemental 6 pin power cable attached, in the primary x16 slot closest to the CPU, try doing the following.
Knowing the model of your previous card and the EXACT model of your power supply would also be helpful. Make certain the card is FULLY seated, and that the lock is engaged in the bottom of the card near the front of the PCI slot.
Power off the unit, switch the PSU off and unplug the PSU cord from either the wall or the power supply.
Remove the motherboard CMOS battery for five minutes. During that five minutes, press the power button on the case for 30 seconds. After the five minutes is up, reinstall the CMOS battery making sure to insert it with the correct side up just as it came out.
Now, plug the power supply cable back in, switch the PSU back on and power up the system. It should display the POST screen and the options to enter CMOS/BIOS setup. Enter the bios setup program and reconfigure the boot settings for either the Windows boot manager or for legacy systems, the drive your OS is installed on if necessary.
Save settings and exit. If the system will POST and boot then you can move forward from there including going back into the bios and configuring any other custom settings you may need to configure such as Memory XMP profile settings, custom fan profile settings or other specific settings you may have previously had configured that were wiped out by resetting the CMOS.