Can I play my hidden card after combat ends and I have no units on battlefield?

No, you cannot play your hidden card in that situation.

According to Rule 461.5.c and related RiftJudge FAQ #4944, once combat ends and you have no units remaining on the battlefield, you lose control of that battlefield. At that point, any hidden cards remaining there are removed and placed in your trash.

The sequence works as follows:

1. Combat Resolution: After combat damage is dealt, the game proceeds to the resolution step (Rule 461).
2. Loss of Control: If you have no units remaining at the battlefield during the combat cleanup/resolution, you lose control of that battlefield (Rule 461.5.b).
3. Removal: Rule 461.5.c specifies that hidden cards are removed from battlefields that you no longer control (or that do not share a controller with your units).

Because this happens during the cleanup phase of combat, the window to play the hidden card closes the moment combat damage concludes and your last unit dies. You do not have an opportunity to react after the combat has fully resolved and you have lost control of the battlefield.