The word otherwise can be used in several different ways:
Otherwise can be used to introduce the possible outcome of a given situation.
We must hurry up; otherwise we will miss the train.
The same idea can be expressed using if not or unless.
If we do not hurry up, we will miss the train.
Unless we hurry up, we will miss the train.
You must work hard; otherwise, you will fail the test.
If you do not work hard, you will fail the test.
Unless you work hard, you will fail the test.
Otherwise can also be used as an ordinary adverb.
You are in love with him. Why do you want to pretend otherwise?
Otherwise can mean if not. It is used for saying that if one thing does not happen, something else (usually bad) will happen.
I hope she mends her ways. Otherwise, she will be sacked. (= If she does not mend her ways, she will be sacked.)
The vaccine has saved tens of thousands of children who would otherwise have died.
Otherwise can also be used to suggest that something is true because the situation will be different if it was not true.
Of course she is interested in him. Otherwise, she wouldn’t be asking about his whereabouts.
She must be very intelligent; otherwise, she wouldn’t have solved this puzzle.
Otherwise can mean differently.
Governments must ban or otherwise prevent the sale of illicit liquor.
Otherwise can also be used to suggest that something is true except for the fact you have just mentioned.
The train was crowded but otherwise the journey was pleasant.
I am a bit tired but otherwise I am all right.