People using Gosport Leisure Centre were unable to book classes or buy tickets to swim from Saturday evening until yesterday.
Anyone who wanted to book a place for a class was not guaranteed one as spaces had to be manually booked by staff on to paper.
The reception tills were also affected.
The problem occurred overnight on Saturday, May 28 until Sunday, May 29 when a main server failed.
But engineers were able to fix the ‘uncommon and unexpected’ failure yesterday after four days of it not working.
James Dumper, general manager at the leisure centre off Forest Way, said: ‘We encountered an unexpected, uncommon and critical systems failure over the weekend which caused some of our bookings and activity systems to go offline.
‘We have back-up systems to minimise data loss and to prevent this being a long-term problem. We have been working with IT engineers to resolve the initial hardware failure.
‘We have been operating a manual system, which has meant there has been no loss of facility access at the centre.
‘But this has now been resolved and all systems are operating as normal.
‘We apologise to all customers who were inconvenienced over the weekend.’
The leisure centre sent an e-mail out to all its members warning them of the fault.
It said it was unrelated to previous booking problems.
‘We understand that there have been previous issues with the online booking system, which we regret and apologise for; this more recent issue is equally as frustrating but unrelated,’ the e-mail said.
It added: ‘As soon as we were made aware of the situation we have been in constant contact with IT engineers who have also visited site to try and reboot the server so we could return to normal.
‘Our reception team are following a manual process of ticket sales for swimming, gym and court bookings etc. They will also be following a fair process for class bookings.’
The leisure centre kept its members updated with the problems on its website.