Что имеем, не храним, потерявши, плачем

We realize what love, friendship, things, etc. mean to us when we lose them. See Цену вещи узнаешь, когда потеряешь (Ц)

Cf: The cow knows not the value of her tail till she has lost it (Am.). The cow knows not what her tail is worth till she has lost (loses) it (Br.). One's blessings are not known until lost (Am.). We know the worth of a thing when we have lost it (Br.). We never appreciate the sunshine and the rainbow until the storm clouds hang low (Am.). When the pinch comes you remember the old shoe (Br.). The worth of a thing is best known by its want (Am.). The worth of a thing is best known by the want of it (Br.). You never miss the water till the well runs dry (Am., Br.)