"I'm in trouble! I lost my keys."
from what I understand, one of the uses of たいへん (大変) can be to express a bad situation, like it's being used here. I'm not sure what the exact translation would be, but I'm also not entirely sure "I'm in trouble" is the closest to the actual meaning. In this context I'm pretty sure it's just expressing something along the lines of "this is bad." (feel free to correct me if I'm wrong on any of this.)
I entered "困っています、鍵を無くしました", because I feel it is a more direct translation. Since 困 = trouble, and 困っています = in trouble.
The translation they show = 大変です！鍵をなくしました。 I would translate to English as; it's serious or terrible, I lost my keys.
Both convey a similar message, but the translation I used, explicitly says the person is in a state of trouble. The translation they show just states that "it" is serious or terrible, and is implied that someone is in a state of trouble.
Not sure how accurate my thoughts are, if at all.