What would happen to cells without the endoplasmic reticulum?

Well it of course depends on which ER you're speaking of. The rough ER has attached ribosomes critical for protein synthesis. Smooth ER produce lipids and steroids etc so basically cells NEED these organelles. read more

From Wiki: The endoplasmic reticulum (ER) is a type of organelle in the cells of eukaryotic organisms that forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known as cisternae. read more

The endoplasmic reticulum serves many functions thus without it all those functions would vanish leaving the cell incapable to manage properly resulting in death. The endoplasmic reticulum is divided into the smooth and rough endoplasmic reticulum; both vary in function. read more

Without ribosomes, not enough proteins would be produced for normal cell functions Golgi Apparatus Proteins would not be packaged which would not allow lysosomes to have digestive enzymes and because they were the garbage can of the cell there would be waste buildup of materials. read more