A phone call to back office, if the contractors are still at work.

If all oneworld check in / ticketing staff have gone home, you wait until the next morning. Potentially (I don't know this for sure) you would not be allowed to go airside again without a ticket and you would have to wait in the international transit area (!)

There are worse airports to do this --- it's safe, although often very cold at PEK T3 --- but unless you have unrestricted entry to China/Beijing, it wouldn't be at the top of my list.