If I am deciphering your post right, you forgot to put the airlock back on. If so, put the air lock on immediately! If there was still active fermentation going on while the air lock is off, the yeast is still producing CO2. CO2 is heavier then air, so hopefully there is a blanket of CO2 on the beer keeping it protected. If not, you could get some oxidation or other contamination from other yeasts/bacteria floating around. Good luck!