Several reasons.1/ You could have scraped the lining of the freezer and opened up a hole in the freezer tubes, even slight pressure on them, which are normally embedded in the insulation behind the lining and therefore lost the freezing gas. 2/ May have forgotten to turn the freezer on again. 3/ Just disturbing the unit can sometimes fracture a freezer pressure pipe or join in the insulation. 4/ If it's a separate unit the fuse or overload on the sealed unit may have open circuited, 5/ Sealed unit may have seized if the gas oil mix was too low or had escaped. 6/ If unit continues to run all the time, the thermostat maybe U/S. 7/ Corrosion of the pipes can also create a slight loss of gas and the cleaning of the ice with the unit being jarred slightly can be another reason.
Look, Listen, Feel and go back over what you did when you decided to defrost it and reverse what you did in the process.
My own experiences due to trade practices over many years.