In Japanese, there are several adverbs that are traditionally used exclusively with negative expressions (containing ない). 全然（ぜんぜん） is one of them. When used at the beginning of a ない (or ません) containing phrase, 全然（ぜんぜん) has the meaning of ‘completely not (A)’, or ‘not (A) at all’.

• やばい、トイレットペーパーが全然（ぜんぜん）ない。
  Oh shoot, I don't have any toilet paper at all.
• 彼女（かのじょ）は家事（かじ）の手伝（てつだ）いを全然（ぜんぜん）しなかったので別（わか）れました。
  I broke up with her because she didn’t help out at all with house chores.

In writing, it is equally as natural to use either 全然（ぜんぜん） as kanji, or hiragana only, so both forms may be used confidently.
ぜんぜん is more flexible (and friendly sounding) in casual conversation than 全（まった）く, a similar expression which has a formal (often negative) nuance.

• 今月（こんげつ）もお金（かね）が全然（ぜんぜん）無（な）いからパンだけを食（た）べる。
  I don't have any money this month either, so I am only eating bread.
• 昨日（きのう）後輩（こうはい）からもらったお土産（みやげ）は全（まった）く美味（おい）しくありませんでしたね。
  The souvenir we got from our junior member yesterday was not tasty at all, don't you think?

While both of these sentences have the meaning of ‘not (A) at all’, まったく could give the listener the impression that you are not pleased with the situation. This will depend quite a bit on tone of voice, though.