Open your Web Part page in Sharepoint Designer.

Step 1 – Remove CSS

Around line 34 you will find a code block like the one below

<SharePoint:UIVersionedContent ID="WebPartPageHideQLStyles" UIVersion="4" runat="server">

<ContentTemplate>

<style type="text/css">

body #s4-leftpanel {

display:none;

}

.s4-ca {

margin-left:0px;

}

</style>

</ContentTemplate>

</SharePoint:UIVersionedContent>

Delete this code block.

Step 2 – Remove the overrides for the left column

Further down you will find three lines which prevents the left column for rendering.

<asp:Content ContentPlaceHolderId="PlaceHolderPageImage" runat="server"></asp:Content>

<asp:Content ContentPlaceHolderId="PlaceHolderNavSpacer" runat="server"></asp:Content>

<asp:Content ContentPlaceHolderId="PlaceHolderLeftNavBar" runat="server"></asp:Content>

Remove all three lines.

Step 3 – Save the page