The Queensway Carleton Hospital's internet and external phone lines went down around 2 p.m. on Friday.
A spokesperson for the hospital said the communications outage was caused by a network hardware failure and that redundancy systems were delayed taking over.
"Staff have moved back to charting on paper by hand, and other emergency downtime procedures, such as communication by radio and emergency phone lines," spokesperson Ann Fuller told CBC in an email.
Fuller said the outage may cause delays in the emergency department with patients waiting longer to be seen.
For families of admitted patients, Fuller said, they will likely have difficulty reaching their family member or their clinical team.
The hospital's Twitter account promises updates on the situation.
Fuller said this was not the first time the hospital had experienced an outage like this, but could not confirm when the last one was.
Their phone lines also went down during the destructive derecho storm the city experienced in May.
This outage follows the "critical" network failure at all three Ottawa Hospital locations last week, where the hospital officials left the situation publicly unexplained for nearly 12 hours.