C01 means no supply in the Hitachi chart consulted. Check that water is available and reaching the machine before attributing the code to a faulty valve.
The match comes from the official Hitachi Home Appliances FAQ. The connection diagram should be checked in the model's local manual.
What changed before the alert
If there was an outage, check that the supply has been restored. If the machine was just installed or moved, check the valve and the outside run of the hose: it may be shut off or pinched even though the rest of the house has water.
Confirm how many inlets your appliance requires. Do not force a dual connection on a single-inlet model or disconnect a hose with the valve open. Cleaning inlet filters should only be done if the manual lists it as user maintenance, with power and water isolated.
If C01 returns with correct external supply, note whether water reaches the drum and request a fill-system inspection. The message does not identify the motor, valve, or sensor separately.
Report whether the alert started after an installation change or with no prior changes; focus the inspection on the water supply.