I just completed the game as a Paragon character. Everyone in my party survived, and chose the good ending. Now I am given the option to either Continue the game, or Import my shepard into a new game with all of my weapons and level. If I were to choose to continue the game with option 1, would I be able to restart it at any point with the same character, or is this my one chance? If I do restart, do I gain all of my upgrades back?

I don't think it's a one-or-the-other kind of thing. It's just that, if you import that ME2 clear save and start a new game+, it will only take into account your character as he/she was at the ending. The continue option is for N7 missions and DLC.

Is there anything special about coming back and continuing? Does it take place after the ending, or before it?

" Is there anything special about coming back and continuing? Does it take place after the ending, or before it? "

Nope. It's after. Your party members reference what you did to the Collector base, and if they agree or not.

I click continue by accident instead of restart can I Restart After I clicked continue

" @Cirdain: Yes you still can. Just create a new game, and you'll have the option of Importing a ME2 character. "

" @Cirdain: Yes you still can. Just create a new game, and you'll have the option of Importing a ME2 character. "

You can't make any changes to your ME2 character on import.  You keep the same face and class.