It’s there but it’s displaying below your posts. The reason your sidebar has fallen and is displaying below your posts is either due to copying and pasting or it can also be a result of importing text containing invalid HTML. Typically this is caused by an open HTML tag in one of your posts.
Go to Settings > Writing and select “ ___ WordPress should correct invalidly nested XHTML automatically” and then scroll down and click “Save Changes.”
Posts > All Posts, change all your latest posts to Draft, click Update, change them back to Published, click Update again. (You can edit all of them at once: tick the square in front of their titles then select Edit from the Bulk Actions dropdown and click Apply.)
If that does not do it then have you copied and pasted into a text widget?