Word Document displays in 2 columns

Assuming that you are opening the document directly from an email attachment, it may be that you are opening in Full Screen Reading view. If it is displaying as two pages, you can opt for a single page instead, or you can also opt not to open attachments in Full Screen Reading view. Both of these option are on the View Options menu in Full Screen Reading View.

If you are in Print Layout view and are seeing two pages side by side at 100%, then unfortunately that is a natural consequence of having a wide screen at a high resolution. You can increase the Zoom until you get back to one page, or you can click on the percentage (beside the Zoom slider) to open the Zoom dialog, then select the radio button for Many Pages and select 1 x 1 Pages from the dropdown. Unfortunately, this setting is easily undone.

If you are truly seeing a document in two columns, then it must be formatted to have two columns; you can see it in one column if you switch to Draft view. If you had Word 2013, then you could well be seeing two columns (as opposed to two pages) in Read Mode (the current version of Full Screen Reading), which has a different display layout; one of the display options is Column Layout.