if i control a battlefield with a hidden zhonyas and a unit. if the unit gets hit by singularity and dies can i recall zhonya's to base

No — not normally. Leave Zhonya's hidden and it's trashed when the unit dies and you lose the battlefield (Rule 323.7). Reveal it in response to Singularity and its replacement effect is mandatory — it kills itself to save the unit (heal, exhaust, recall it to base), so Zhonya's goes to the trash, not your base.

The one exception: if the unit has Deathknell (or another death trigger that forms a chain), you get a window after it's already dead. The trigger goes on the chain during cleanup; reveal the hidden Zhonya's then. It won't save the dead unit, and as face-up unattached gear it's recalled to your base in that same cleanup (Rule 457). Caveat: the Zhonya's must have been hidden on a previous turn — hidden cards have no Reaction on the turn they were hidden (Rule 811.1.b).