Our machines with higher end graphics cards and multiple ports usually have multiple screen configurations.
For example, if you have an HDMI port, you could have all of these configs available:
laptop screen only
laptop + HDMI clone
laptop + HDMI extend screen
I've even run into a few machines that had "both screens off" as an option.
If you have a VGA card AND HDMI, it gets even more complex with the same combinations of options for VGA and often with the same combinations but as a VGA/HDMI output with the laptop screen off.
Unfortunately, most laptop companies only give us a single key to manage this. It's a repeating toggle where each time you press it, the screen cycles to the next combination.....and they can be in ANY order.
Worse yet, that key is often different on multiple machines. On my Fujitsu it is Fn + f10. On my Dell, it is Fn + f8.
It is almost always a Fn + Function key though. (Fn is usually grouped with the lower left Alt, Windows, and CTRL keys)
Because it can be so complicated, most machines will automatically switch when plugged in. But what they switch too can differ from machines as well.
My bet is that when you plugged it in, it switched to HDMI and shut off the screen. If so, you should be able to look a the function keys at the top of the keyboard and find an icon that looks like a screen, or multiple screens, or a computer and a TV, etc. (They're all different as well). If memory serves me right, I've always seen it between f7 and f12 and almost never on f1 to f6. Other icons should be self explanatory such as screen brightness, volume, mute, wifi, mouse, etc.
Mke sure the cable is out of the port and boot the computer. Wait a while for it to be completely booted. (you should hear it slow down). Locate the screen function key and press it while holding down he Fn key. Give it a few seconds and if nothing, try again. You may need to do it several times to cycle through to your normal laptop screen output.
That should fix you.
As for the output not picking up on the HDMI display, it's possible the wrong input was selected. Many will have 2-4 HDMI ports and if you are set on input one and plug into input two, you won't see anything. But the computer will know and will be very helpful and will switch the screen for you anyways which I think is what happened here.
Of course, this is the simplest and most likely solution as just about everyone in my family has called me about this at some point. If that doesn't get it, get back. Good luck!