Sidebar is at the Bottom

Hi all. I am attempting to write a post but the preview is showing the sidebar (to the right) below my text. I deleted the post and tried to start again but even if I just write the title – no text at all in text box and hit ‘save and continue editing’, the sidebar appears below everything. I don’t understand why this is happening. Any help would be great as I want to publish a post now. Thanks guys.