How do i go about companys taking money out of my account?

i checked my account this morning and had $30 some dollars in my account go to get my son diapers and get told oh you don't have enough money in your account for this so i called my bank they gave me the name and number of these 2 places who took money out of my account they we're esavers and emi encore money marketing... how do i get my money back?! what do i have to do?