http://revach.net/article.php?id=4656

Section: Tanach   Category: Eichah
Megilas Eicha: Lechem Dima - The Question And Answer Behind The Word Eicha
Chazal say the word Eicha is two words "Ei Koi", what happened to the promise of Koi.  The first Koi is the promise to Avrohom Avinu "Koi Yihiyeh Zaracha" meaning your children will be like the stars (Lech Licha 15:5).  The second is "Koi Soimar L'Bais Yaakov" (Yisro 18:3) which Hashem said at Matan Torah.

The Lechem Dima (Rav Shmuel D'Uzida Talmid HaArizal) says that the word Eicha is asking, "Ei Koi", what happened to the promise of multitudes, "Yashva Badad" and why is Klal Yisroel sitting alone decimated and demoralized?  The answer lies in the second Koi, as Hashem asks "Ei Koi" what happened to the promise Bnei Yisroel made at Har Sinai of Naaseh ViNishma.  When we keep ours, Hashem will certainly keep his.