This is most probably an inadvertent change in your default paragraph settings.

Open a new document and type Alt+O,P or otherwise open your paragraph dialogue box. If the line spacing (lower right of box) shows Double, you can change it to single. However, it will remain double in the Normal.dot template on which your new documents are based.

Type Alt+O,S to open the Styles pane on the right of your document. The Normal paragraph style will have a bue box around it, indicating that this is the style--i.e., set of font, size, line-spacing, etc. presets--applied. Right click on the name Normal and select Modify. In then ensuing dialogue box, click on the Format button, bottom left, and select Paragraph. In the line spacing, select single and click OK. Then, before closing the Modify Style box, at the bottom select New documents based on this template.