Never realized until a colleague of mine asked me that it wasn't easy to un-hide the column(s) and retrieve the filled up/ edited information in MS Project.

The bitter fact of the matter is that...  in case you need to un-hide a particular information column...

You will have to insert the required column (MS Project doesn't have an "unhide column" option). Right-click on an existing column heading and choose "insert column" option. After this, select the column/field you want. The new column will appear next to the column you had chosen.

If you plan to temporarily hide a column, do not use the "hide column" option since you will lose the custom headings that you may have defined (the data remains intact of course). Instead, using your mouse, click the column boundary and move it from right to left till the column is hidden. To unhide, drag from left to right.  [Source:  http://www.gantthead.com/discussions/discussionsTopicContainer.cfm?ID=5099]

Tried the following link too to understand...
http://www.ehow.com/how_5079889_unhide-columns-microsoft-project.html

Hope you guys using the software will not make mistake of hiding the columns permanently, which you may desire to use otherwise.