Learn English – “When would be convenient for you to meet?” VS “When it will be convenient for you?”

So you have two choices:

Not after midnight.
Not until midnight.

After: later than something; following something in time
Until : up to the point in time or the event mentioned

If you say "Not after", you are saying that the restaurant doesn't close later than midnight. It could close any time before midnight, so you haven't answered the question "What time does the restaurant close?". You have said when it doesn't close.

If you say "Not until", you are saying that the restaurant doesn't close up to the time of midnight, then at midnight, it closes. That does answer the question "What time does the restaurant close?".

Number 1 is correct.

When we construct a statement -- not a question -- in English, we normally phrase it subject / verb / object. Like "You / go / to the store." When there's a "helping verb", like "will" or "do", it goes with the verb. "You / will go / to the store."

But when we construct a question in English, it's normally "interrogative word / helping verb / subject / regular verb / object". By "interrogative word" I mean a word like who, when, where, how, etc. So, "Where / will / you go?" "Why / did / Bob / leave?" Etc.