Harry and Ron were only in the vicinity of the platform entrance for about a minute, and left within seconds of the train leaving the platform, a time when we could reasonably assume most parents were still waving their kids off.
Since Dobby was evidently in the immediate area and using magic to prevent the portal from activating, it's quite reasonable to presume that as soon as they'd left he either removed his charm or simply stopped casting whatever spell was used, thus allowing the portal to re-activate. Those on the other side probably didn't even notice that anything was amiss. Since we simply don't knowsimply don't know what magic he used, anything beyond that is pure guesswork.
Professor McGonagall suggests that in the event of an emergency, the appropriate thing to do is to alert the school. Again, beyond that we simply can't even guess how the problem was rectified:
… so we had no choice, Professor, we couldn’t get on the train.’
‘Why didn’t you send us a letter by owl? I believe you have an owl?’ Professor McGonagall said coldly to Harry.
Harry gaped at her. Now she said it, that seemed the obvious thing to have done. ‘I – I didn’t think –’
‘That,’ said Professor McGonagall, ‘is obvious.’
Clearly it's resolved by the following year, though.
