West Mercia Police had alerted people to its call centre being unable to answer emergencies from around 4.30pm on Monday afternoon.
The force updated people saying that system was back up and running on Tuesday morning, around 15 hours after the first message.
It has been confirmed that the incident was caused by a power failure – which is now being investigated.
It is the third time in recent weeks that the force's 999 system has failed, with the matter being raised at a meeting of the West Mercia Police and Crime Panel only last week.
In the event of a failure 999 calls are still answered, but are handled by staff from other forces who step in to cover the gap.
West Mercia's Police and Crime Commissioner, John Campion, said the successive failures were "simply unacceptable".
A spokesman for West Mercia Police said that contingency arrangements had worked, and staff were able to respond "to high priority incidents".
He said: "We experienced a major power failure at around 4.30pm on Monday afternoon which limited our ability to access some systems and manage phone calls for a number of hours.
"Contingency arrangements with neighbouring forces ensured that emergency calls were still received and officers and staff continued to respond to high-priority incidents, protecting the public from harm.
"We are now handling our own 999 and 101 calls, but would continue to ask the public to report non-urgent matters via our website.
"The cause of the power failure is under investigation and we are working closely with the Police and Crime Commissioner."
Mr Campion said: “The public must be able to access their police in an emergency, and my work as commissioner is focused on ensuring they can.
"This outage is the third in a matter of weeks and is simply unacceptable.
"Whilst I am reassured that contingencies worked as they should, and officers and staff stepped up, I recognise the significant impacts this can have on communities – most importantly around safety and confidence.
“I will continue to hold the Chief Constable to account for the vital emergency service West Mercia police delivers, and I will support the force to identify a solution and provide further investment where needed.”