I assume you have had an email advising of the failure of the direct debit. The next email you should get will be a "Payment Reminder" that you need to pay within 14 days. That email will include the following information:
If you've got any problems with making a payment just call us on 0800 432 0200.
If you have not had that email then the on line link may not yet work, but I would try it asap..
Plusnet's system is supposed to allow you 14 days to pay but it can be somewhat erratic in what actually happens - I should know as my last direct debit failed (due to Plusnet's appalling billing system that had not worked properly for 19 months when all was supposedly set up correctly with my bank). However, the system almost immediately locked me out of my account meaning I could not access that payment link and it took three staff members to sort things out (after I phoned in), taking two hours to actually let me in after the missing amount had been credited to my account.