How do i get air back into my whipped cream can?

someone must've been holding the whipped-cream 'aerosol' can upright and let all the air out, now my whip cream comes out runny, and sometimes it doesn't even come out but i can hear it sloshing in there. how do i get the air back inside it to pressurize it or whatever to make it come out whipped again? no point wasting a good can of whipped cream lol