Can I re-carb this beer?

I was near the end of a kegged SchwartzBier when I ran out of Co2. There
was about a gallon or so left. I thought my son poured the remaining gallon
into a pitcher or 2 and finished it off, but he only poured off a little. He left the top off of the corney and now it’s completely flat.

Can I just re-carb this now that I have a fresh supply of C02 ?

Sure you can, though with the lid off I’d be concerned about the beer being oxidized or infected. If it tastes okay though I’d go for it.