(CNN) A ransomware attack has crippled an estimated 400 dental practices across the US.

"We have no access to the patient charts, schedule, x-rays, or payment ledger," Shae Johnson, the Clinical Coordinator at Dentistry Design in McFarland, Wisconsin, told CNN. "The doctor cannot do proper treatment without a chart history and x-rays."

Ransomware is a type of virus that locks up a computer and demands a payment for it to be unlocked.

On Tuesday, two Wisconsin companies that provide an online service to dentists' offices, Percsoft and the Digital Dental Record, told their customers that the software they use to connect to individual offices had been infected with ransomware Monday morning. In periodic updates, they say they are working on a case-by-case basis.

"Essentially the restorations are ongoing," Brenna Sadler, a spokeswoman for Digital Dental Record, told CNN. "It's a very difficult, lengthy, methodical process. So it's taking some time."

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