Colloquial contraction of "なければ" used to mean "if (something) does not exist / if one does not have ..." (conditional).
お金がなきゃ旅行には行けない。
If I don't have money, I can't go on a trip.
材料がなきゃこの料理は作れないよ。
If there aren't the ingredients, you can't make this dish.
免許がなきゃ車を運転してはいけない。
If you don't have a license, you mustn't drive a car.
Colloquial shortened form of "なければ" used as a conditional ('unless' / 'if not ...').
明日雨が降らなきゃ、ピクニックに行く予定だ。
Unless it rains tomorrow, we plan to go on a picnic.
約束を守らなきゃ、信用を失ってしまう。
If you don't keep your promises, you'll lose trust.
電池がなきゃリモコンは動かない。
Unless there are batteries, the remote won't work.
Colloquial abbreviation of "なければならない" meaning "have to..." or "must..." (expresses obligation or necessity).
もう行かなきゃ、遅刻しちゃう。
I have to go now, or I'll be late.
宿題をやらなきゃ明日の授業に出せないよ。
You have to do your homework, otherwise you can't hand it in tomorrow's class.
早く寝なきゃ、明日つらいよ。
You must go to bed early or it'll be tough tomorrow.