A power will not cast

1. Cost. Do you have the mana, stamina or health it needs? Flying makes every power cost 1.5×.
2. Cooldown. Is its icon still dark? Hover it for the seconds left.
3. Target. Is your target something this power can affect, and in range? Aimed powers use what is under the crosshair first.
4. Stance and equipment. Some powers need a particular weapon or stance; the tooltip says which.
5. Control effects. Are you stunned or silenced? Check your effects bar.
6. The ground. Are you, or your target, on dead magic ground? Nobody casts there. See Magic zones: low and dead ground.

The chat log usually says why. A cast the server refuses costs you nothing.