Further disruption was expected today unless the computer system, which serves all the board's hospitals was successfully repaired overnight, after it suddenly stopped working yesterday morning.
Doctors, nurses and administration staff were left unable to access vital clinical information, including patients records.
The board said that a total of 288 outpatient appointments, along with four planned inpatient procedures, 23 day-cases and 40 chemotherapy treatments had had to be put back because of the problem.
A spokeswoman said yesterday: "We are trying everything we can to fix the issue but there is a chance that it could run into another day."
NHS Greater Glasgow and Clyde added: "Our technical staff are working flat out to resolve this.
"However as a result, we have had to postpone a number of operations, chemotherapy sessions and outpatient appointments.
"There was also some delay in calls to our switchboard being answered.
"The problem relates to our networks and the way staff can connect to some of our clinical and administrative systems."
The board said that those affected could be assured that their care would be rescheduled at the earliest opportunity.
"We are extremely sorry for the inconvenience," the statement added.
Scottish Conservative health spokesman Jackson Carlaw said: "While we acknowledge that NHS GG&C has been quick to admit to a serious problem, it needs to resolve the issue urgently and urgently reassure the public that the chaos of Tuesday will not be repeated.
"Inevitably patients will be concerned and rightly so - this failure will have been distressing for many.
"The Scottish Government must stand ready to help and ensure other health boards have all the information they need to avoid a similar crisis elsewhere."