On the dates HHN is happening, the park closes to the public at 5:00 for the studios.
If you are not in the park by 4:59/5:00, you have to wait outside the gate until the park is open for hhn.
I always make sure I am at the the park entrance line by 4:45 to enter the park.
Some dates the line can be long and you don't reach the ticket gate until close to 5:00
If you have a day park ticket and hhn ticket, you are allowed to stay in the park and wait it out in the holding zones until hhn starts.
If it is 5:00 and you have the park day and hhn ticket, you won't be allowed into the park if you haven't cleared security.
You would have to wait until till they reopen the park to enter.
It varies each year if any of the holding areas are released before the others.
Some years it has been Finnegans while other times the holding pen near Hello Kitty was released first.
I usually go to the Simpsons holding area as there are plenty of seats in that section.