28 deg is fine but in future I suggest using a big spoon to stir very vigorously to add back enough O2 into the wort, I do it for at least 2 min.
The boiling process removes all the O2 so you need to add some back because the yeast need it to divide and create more yeast.
By the way thats the only time you want to stir and splash things around everywhere else in the process O2 is bad.
Also have you checked for leaks sometimes the lid will not seal properly and it seems like nothing is happening when it actually is.
But it can take up to 2 days for fermentation to start before I would start wondering whats wrong.