Sounds like it could be "space dust"; a small entity or entities that ends up a great distance from the main drawing. Do a "Ctrl+A" to select all the entities and see where the bounding box is situated. Hold the Shift key and drag a selection window where the drawing should be; thus, deselecting those objects. Check what is left selected in the Selection Info palette. Delete, or not. Hopefully your zoom will return to normal, if it turns out to be space dust.