As far as I can tell, there's no way to re-straighten an object after rotating it. There's nothing retaining what the degree of rotation was. That would be really helpful.
Select two point to rotate an object so these point will be lying on a horizontal line.
There are some ways to do it, with limitations.
You might want to try Quick Orient tool from AstuteGraphics plugin pack, or scripts, like the http://vectorboom.com/load/freebies/freescripts/how_to_reset_text_objects_and_bitmap_images_to_horizontal_position_in_illustrator/22-1-0-368 (for text and images), or rare Horizoner script ↑