If you don't fix it quickly it'll get damaged by water leaking in through the gap when it rains.
You better get it fixed soon or it will break from water leaking in through the gap when it rains.
確かにその場合には出来るだけ早く直してもらった方がいいですよね。
早く直さないと ＝ If you don't fix it quickly (then) ...
又は You better get it fixed soon or ... (文字通りですと＝＞早く直してもらった方がいいよ、しないと。。。）
雨降ったら ＝ when it rains
隙間から = through the gap; through the crack
（水）は侵入して故障する = (water) will leak in through the gap and damage it;
(it will=>) it'll get damaged by water leaking in
it will break from water leaking in
アメリカ合衆国 
