Simple Excel sort did not work correctly

[Spideriffic]

Hello,

I am using Microsoft Office Home and Student 2010 version 14.0.6123.5001 (32 bit Dell computer running Windows 7). I have a simple worksheet with data about my customers - first name, last name, phone, email, amount paid, etc. Each line represents one customer and all of his or her information. I entered data in no particular order, and then attempted to sort the data by last name. I did this by clicking on the top cell in the last name column, then I clicked on "Sort and Filter", then "Sort A to Z". I have successfully used this technique in the past to sort a worksheet successfully. It would cause the entire line to move to a different order, and each customer's data stayed together. However, recently I tried to do this, and only some of the columns sorted correctly. In other words, data from one customer ended up on a different customer's line. What a mess! This is taking me hours to manually straighten it out. I realize that I could have hit undo and straightened it out in one second, but I didn't notice the problem until much later after lots of new data had been entered.

I'm trying to determine what I did wrong so that the error will never happen again. I'm wondering if it has something to do with column filters being used during the sort. If my data was filtered, for example, only showing customers from a particular area code, would that mess up the sort? Or, does having some merged cells in the worksheet, or hidden columns, affect the sorting process? Does the correct technique to sort the entire sheet involve highlighting an entire column, or the entire sheet?

I realize that this is "Excel 101", but I've never had this problem before and I hope that someone here can advise me.

Thanks very much!