一旦（いったん）, an adverb meaning 'for a short time', or 'briefly', is a structure that is used in Japanese to indicate that 'once (A), (B)'. This particular meaning comes from the inclusion of hypothetical structures such as ば, と, or たら, and will usually appear with a negated verb in the (B) part of a sentence.
一旦（いったん） based sentences will indicate that returning to the previous state will be difficult, at least for a short time.
Other common translations include 'for the time being', and 'for now'.
一旦（いったん） will primarily appear at the beginning of sentences, or directly before a ば, と, or たら modified verb.
一旦（いったん）再起動（さいきどう）をすれば、もとに戻（もど）るはずです。
Once the system is rebooted, it should return to normal.
娘（むすめ）は一旦（いったん）泣（な）き出（だ）したら、しばらく泣（な）き止（や）まないので大変（たいへん）です。
Once my daughter starts crying, she wont stop for a while, so it can get hard.
一旦（いったん）集中力（しゅうちゅうりょく）が切（き）れてしまうと、再（ふたた）び集中（しゅうちゅう）するのに時間（じかん）がかかる。
Once I lose focus, it takes me a while to regain my focus.
Fun-fact - 一旦（いったん） originally comes from the noun 一旦（いったん） meaning 'one morning', or 'one brief period'. The 旦（たん） kanji itself signifies 'daybreak' or 'dawn'. In this way, it can be a useful memory tool to remember 一旦（いったん） as the 'dawning' of some period that will not end until it has run its own natural course.