Rashi: When I saw my days cut off, desolate in gloom, like "Nidmah Kol Am Kena'ani" (Tzefanyah 1:11). I.e. when I was ill, I thought that I will go to the gates of the grave; now I will die, for until that day, no Choleh healed ¹ (Pirkei d'R. Eliezer 52).

Radak: It is the cutting off of my days. Chizkiyah was only 39 at the time. The same applies to "v'Damisi" (Hoshe'a 4:5), "Oy Li Ki Nidmeisi" (above, 6:5) and similar verses. When the Navi told me that I will die, I thought, how will I go to the grave before my time, and my years are cut off? How are my other years lacking - most people live more than 50 years! Chazal said (Mo'ed Katan 28a) that one who dies before 50, this is Kares.

Malbim: He depicts time as something that moves, from birth until death; then his days are silent. Time is nothing; it is only a means of obtaining things. When a matter ceases, also the time ceases, for it was only a measure of its existence. When a person died, before the body decays, he is still tied to time, just his days are quiet and do not move. The movement of days is only when the body moves, in his lifetime. "Elchah b'Sha'arei She'ol" - before the grave will consume my body, I will be with my silent, ceasing days.

Rashi: This is like "v'Lo Nifkad Mimenu Ish" (Bamidbar 31:49); my other years were lacking.

Rashi citing Targum Yonasan: You remembered me for good; You added to my years. You visited me for good, and added to my years.

Malbim: After the grave will consume my body, then the time allotted to me is totally void, and lacking from the years of my life.