There are four stages to park closing:
Stage 1 - The parking lot entrance is closed. You can only access the park if you are using Disney Transportation.
Stage 2 - The ticket windows are all closed (and the automated Ticket Vending Machines are turned off). Unless you already have a ticket you cannot get in.
Stage 3 - Only people with Annual Passes or current Resort IDs will be allowed in.
Stage 4 - Nobody is allowed in.
Note - having restaurant reservations in a park, even with written proof, will not override a stage closing for you.