The vice-chancellor of Sunderland University said "we just don't know" when disruption caused by a "major cyber-attack" will be fixed.
Telephone, website and IT systems at the university have been offline for several days.
In a video message on a temporary website, Sir David Bell said the attack was "particularly upsetting" as the university recovered from the pandemic.
"We are working as hard we can to resolve the situation," he said.
He said students and teachers had "adjusted well" to life after the Covid-19 lockdowns and "campuses have been much more vibrant".
"So it has been particularly upsetting that our university has been hit by a major cyber attack," the vice-chancellor said, adding: "As a result, we've experienced extensive interruption to our IT systems and services for the last few days.
"The obvious question is how long will this situation last? The honest answer at the moment is we just don't know."
He said "outside experts" were being consulted and the university was "doing all we can" to maintain teaching and "protect the data we hold".
"We want students to continue to come on to our campuses so we can maintain teaching and learning," he said, adding: "It might take a little time but we will get through this together.
Newcastle and Northumbria Universities were targeted by hackers in September last year as the National Cyber Security Centre warned of a spike in attacks on educational institutions.