Sunderland University has been hit by "extensive IT disruption" which has "all the hallmarks of a cyber-attack".
Telephone, website and IT systems are down but face-to-face teaching would continue as far as possible, it said.
The university was working with police and other agencies to find out what happened.
"We take the security of our systems extremely seriously and will work to resolve the situation as quickly as possible," a spokesperson said.
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.
All online lectures due to operate through the university's systems were cancelled.
Sunderland University said students were being encouraged to come on to its campuses where face-to-face teaching was continuing.
"For those studying further afield, plans have been made to ensure they can continue doing so, with minimum disruption," it added.
Its website said it was undergoing "essential maintenance".
❗ University update❗— University of Sunderland (@sunderlanduni) October 13, 2021
The University continues to experience extensive IT issues which has all the hallmarks of a cyber-attack. Our telephone lines, website and IT systems are still down. @Graemethedean @sunderlandsu @UoSFutures @UniOfSunComms pic.twitter.com/2h21JLHib6