What kind of modifier was it? The only one which can be easily reversed once you apply it is Subdivision Surface. Once you apply a modifier the only way to reverse that process is to either undo the operation in the same Blender session history (CTRL+Z) or have a saved copy of the model without the modifier applied. Once you apply a modifier, the application of those changes to the mesh is permanent. It's always wise to save several versions of a model's history to different files so you could refer back to a previous iteration of the mesh if need be.

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