This is the 254th most frequent Polish word.
brak
"Lack" or "absence."
Here, 'brak' signifies an absence of something, in this case, energy.
Nie odczuwam żadnego braku energii.
I do not feel any lack of energy.
Here, 'brak' refers to the absence of keys causing a problem.
Brak kluczy uniemożliwił mi wejście do mieszkania.
The lack of keys prevented me from entering the apartment.
In this context, 'brak' describes the lack of a response and its emotional impact.