What if i have helm of power. and my opponent wants to play a hidden but has no energy left?

No — they can't play it from Hidden without the energy.

Assuming you mean Helm of Suppression: Hidden sets the card's base cost to 0, but the Helm's tax is a cost increase applied afterward (Rule 356.1.b → 356.3). If the Helm is empowered, the final cost is 1 Energy + 1 Power; if not, just 1 Energy (FAQ #11827). Costs are paid in step 4 (Rule 357) and checked in step 5 (Rule 358.2) — if they can't pay, the play attempt is undone and the card stays facedown (Rule 358.5).

One caveat: they could still afford it if they have a Reaction "Add" ability (like a rune seal) that they can activate while paying costs to generate the needed Energy (Rule 357.1.a). Otherwise the hidden card simply doesn't come out.