If flow from taps is OK & no one's been messing around closing isolating valves then perhaps cistern fill valve may be stuck in closed position or clogged with scale or other debris. Might be possible to free it up though - push ball float down if its stuck in the "full" position and if water doesn't flow, try pulling valve activator outward. If nothing happens, unscrew the valve body slowly till water (hopefully) starts leaking out.
Sorry if this is too obvious a solution & you've already tried it.
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