In the darkest nights, the moon is dearly missed

This is the translation of an Arabic proverb which means that during these hard times you miss someone who made life easier, happier, and so on:

In the darkest nights, the moon is dearly missed.

For example, it was first said when a just and kind Arab Leader was killed, and his people became hungry, oppressed, exiled, etc. with the tyranny that replaced it. So basically in these times of hardship and struggle, that man is dearly missed.

Are there any proverbs or saying or lines from poetry that are similar?

Solution 1:

You never know what you've got till it's gone — Wiktionary

Good friends, family and acquaintances shouldn't be taken for granted.

Never take someone (or something) for granted — TFD

To expect someone or something to be always available to serve in some way without thanks or recognition; to value someone or something too lightly.
"I wish you didn't take me for granted."
"I guess that I take a lot of things for granted."

Related: "Let Her Go" by Passenger is a song full of such lines. From azlyrics.com,

Well you only need the light when it's burning low
Only miss the sun when it starts to snow
Only know you love her when you let her go

Only know you've been high when you're feeling low
Only hate the road when you're missing home
Only know you love her when you let her go

Solution 2:

Gregory Titelman, Random House Dictionary of Popular Proverbs and Sayings (1996) has this entry for "You never miss the water till the well runs dry," which seems very much on point:

You never miss the water till the well runs dry. You don't realize how much you have until you lose it. The proverb is of Scottish origin and in about 1628 it was included in James Carmichael's Collection of Scottish Proverbs. In 1746, it appeared in Benjamin Franklin's Poor Richard's Almanac. The proverb is found in varying forms: We only know the worth of water when the well is dry; You don't know what you've got till it's gone, etc. The main entry is listed in major dictionaries of American proverbs. [Individual citations omitted.]

Solution 3:

Lines 7, 8, and 9 of When Lilacs Last in the Dooryard Bloom'd, Walt Whitman’s poetic elegy to Abraham Lincoln written following the President’s assassination in 1865 seem similar in that Venus (the bright western star = Lincoln) disappeared after the assassination (although this Wikipedia article on the poem/elegy offers two alternative interpretations of the lines that do not equate the fallen star with the fallen leader):

O powerful, western, fallen star!
O shades of night! O moody, tearful night!
O great star disappear'd! O the black murk that hides the star!

(first link above to the poem’s full text leads to PoemHunter.com)