Residents of St George's Park in Telford were "not safe", at risk of abuse and occasionally left without medicine and adequate food and drink, the government report found.
Some bedrooms had not been cleaned properly for a month, it said.
Rotherwood Healthcare apologised in a statement to the Shropshire Star.
The Care Quality Commission will inspect the home in six months, but if improvements have not been made it faces losing its licence.
The home, which cares for 60 people with dementia and nursing needs, was visited in November.
Inspectors found a poorly-run home, with staff so hard-pressed residents were left in their beds for hours.
"People were shouting for help from their bedrooms to get out of bed and we had to seek staff members to provide support for these people on a number of occasions," inspectors said.
"One person told us, 'you can't put a time on it but I hope I am up before 11am'."
They found one resident "visibly in pain" waiting days for medicine and others had "appeared to have lost weight" and "showed signs of dehydration".
Some areas of the home were "visibly dirty" with a strong smell of urine, the report said.
The manager of the home resigned in August, and while a replacement had been found, they were not yet in position, inspectors said.
The CQC said if standards had not improved by its next visit, "we will take action in line with our enforcement procedures to begin the process of preventing the provider from operating this service".
Rotherwood Healthcare said it had appointed a private company to help turn around the home.

The victim was walking with two female friends when a verbal altercation took place with a large group of youths. He was then assaulted.
The incident took place in Finella Gardens, in the Fintry area, at about 20:30 on Tuesday 9 February.
The man suffered a neck injury during the assault but did not require medical attention.
Officers said they were keen to speak to two men aged about 17.
One was described as being 5ft 5in, with short blonde hair, and wearing a Nike light grey jogging bottoms.
The other was about 5ft, with short dark hair, and was wearing black gloves and jogging bottoms.
A care home has been rated "inadequate" and put into special measures after inspectors found a string of failures during an unannounced visit.
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