I'm not sure if it's broken or became loose... I was pulling on my front wheel bike lever and I pulled it hard just to check it's condition... it did some sort of a click sound or snap sound (didn't sound like something broke, sounded more like something unlatched?) now the brake lever is SUPER loose and doesn't engage unless it's really pulled down a lot (there's no tension). What did I do? Is this ever a common occurance?

Check your quick releases. If you have campy levers- the QR is on the lever itself.

Quick releases are cammed such that they shoudn't pop open on their own. My guess would be the cable fixing bolt wasn't tight enough and your brake cable slipped. In that case, the cable tension needs to be readjusted. Be glad it happened during a test and not during an emergency stop, and make sure they're nice and tight in the future.

Yes, the cable slipped. Re-tighten and adjust.

If that's not the case, the head may have pulled off the cable