single.php is acting weird

this is the original nav-above code from single.php:

<div id="nav-above" class="navigation">
					<div class="nav-previous"><?php previous_post_link( '%link', '<span class="meta-nav">' . _x( '&larr;', 'Previous post link', 'twentyten' ) . '</span> %title' ); ?></div>
					<div class="nav-next"><?php next_post_link( '%link', '%title <span class="meta-nav">' . _x( '&rarr;', 'Next post link', 'twentyten' ) . '</span>' ); ?></div>
				</div><!-- #nav-above -->

and this is what you made out of it (or what appears in the html code in the browser:

<div id="nav-above" class="navigation">
					</div>
				</div><!-- #nav-above -->

as you can see, there is an unbalanced </div> in it, which causes part of your problem.

remove this </div>, or if you don’t want the next/prev post navigation at all, remove the whole code.

the same with the nav-below code.

this is only part of the problem, because when you remove these extra </div>, the content will go back to normal, but the sidebar will be below the content.
there are now two closing divs missing before the call of the sidebar (and i can’t see yet where these should be).

can you paste the code of single.php into a http://wordpress.pastebin.com/ and post the link to it here?

do you remember having modified other templates as well?
comments.php?

independant from all this:
it is not advised to modify Twenty Ten directly, as this is the default theme of wp3 and will be overwritten with the next update of wordpress; and all your modifications will be lost.
Strongly consider creating a child theme: http://codex.ww.wp.xz.cn/Child_Themes

and make regular backup copies of your theme and database