What is the meaning of "Negasheshah cha'Ivrim [Kir]"?
Radak: This is like Nemasheshah, like "Memashesh ba'Tzaharayim" (Devarim 28:29). The afflictions in Galus are like darkness and blindness.
Malbim: Aside from darkness, we are blind from seeing. I.e. the light of salvation and Divine influence from above are lacking, and also we are not prepared to receive this light, even if it would exist.
Why does it say "uch'Ein Einayim Negasheshah"?
Malbim: An Iver's (blind person's) eyes were blinded. Ein Einayim never had eyes. An Iver can grope around alleyways of the city; he saw them before he was blinded. We are like one who never had eyes.
Why does it say "Kashalnu va'Tzaharayim ka'Neshef"?
Malbim: When an Iver goes during the day, someone else can show the way to him. We stumbled in Tzaharayim like at night; there was no one to show the way to us.
What are "ba'Ashmanim [ka'Mesim]"?
Rashi citing Menachem: It is an expression of Machashachim (dark places). Most explain like this.
Rashi citing Dunash: It is an expression of Shuman (fat); the Aleph is extra, like the Aleph in "Achavasi" (Iyov 13:17), "Achzav" (Yirmeyah 15:18), "Derech ha'Asarim" (Bamidbar 21:1). We are among the living fat people, we are like dead.
Rashi citing Targum Yonasan: It is closed in front of us, like a grave is closed in front of a Mes.
Radak: We are considered as if we are buried, like the dead. My father explained, the Nun in ba'Ashmanim is extra; the root is Asham, like "Tesham Shomron" (Hoshe'a 14:1), an expression of desolation. Graves are a place of desolation.
Malbim: We lie like the dead, and do not move from our place.