One feature of datasheet view is that individual users can, if they wish, swop the column order. Next time the datasheet is opened that new order is retained.
So all you need to do is swop the columns back and possibly save.
Thanks for your reply Colin. I am the only user of the database and I do not change the column order. It happened again on Friday and also sorted itself out again after I opened and closed the database a few times. Bizarre and frustrating in equal measures.
Its easy to click on a header & move it possibly by mistake then forget you've done so! At least that's what I always assume when it happens to me