No Display Signal could mean a number of things... remember even if the System isn't POST-ing the Graphics Card, Processor and Case Fans will still be running.
Now the Gigabyte B450 PRO doesn't have a POST LED., which are incredibly useful for diagnosing boot problems; so instead you'll have to be a bit "Old School" and connect a PC Speaker.
Either you'll have one in the Motherboard Box, or your PC Case should have one... if not and you can get one (they're like £1-2), then a ghetto solution is to use an old pair of headphones; just strip the wires, connect the Red to Power and the Black to Ground; it'll beep at you.
What you're listening for is a single "Beep" after a few moments, this means the POST has worked and the System itself is "Fine" ... at least at a basic level.
If on the other hand you hear something different, well depending on the Series of Beeps depends on what's failed.
POST Beep Codes List
You can use that list to determine what the issue is.
Gigabyte use the AMI BIOS.
Now assuming that's all fine... the next question is WHAT is the Exact Display, as the issue could be that there is a HDMI Signal and it is being sent to the Display BUT the Display itself isn't Automatically Calibrating to accept the Signal.
A good example being my new Display (Samsung RU7470), would not display VESA VGA (i.e. nothing until it loaded the Windows 10 Graphics Driver) without enabling Input Signal Plus.
At least not after I reset the Display to Factory Settings, where-as "Out-of-the-Box" it was perfectly find Auto-Detecting all of the Connected Consoles and PC.
So a possibility is you might need to manually use the Display Menu to set the Source Signal (and Signal Type).
Another possibility and this was a weird issue I had with my R7 360 on my Brother' Rig for a while., was that for some reason the HDMI weren't initializing until in Windows; and even then as a Secondary Display... so we had to use the DVI Port with the DVI-to-HDMI Converter Plug, just to get into Windows to reset the Drivers (to Older Version); which also seemed to fix the issues in BIOS as well.
Of course an alternative issue might be, you've accidentally bricked the RAM / CPU / GPU via a Static Shock.
It's uncommon, but not properly grounding yourself while handling Components means it's a possibility and it often does destroy components as a result.
