BUMMER! I used Crazy Glue on my 15 inch square when it broke in half. I laid it upside down on the table and put a piece of clear packing tape on the crack. I then turned it right side up and used the packing tape like a hinge as I put a bead of Crazy Glue along the crack. I immediately put the ruler flat on the table and held it flat until the glue dried. I am still using the ruler since I posted about it 2 years ago?
I would suggest the clear packing tape on the back where yours is broken as well. I figured if the packing tape stuck with the glue, it wouldn't effect seeing through the ruler. After you have the packing tape on and the ruler looks aligned, bend it open just a bit being careful not to snap the other end and run a bead of Crazy Glue along the crack. I did mine on my old arbourite table in case I had a glue over run but I didn't need it with the packing tape on the back. Hold the edges fiat on the table until the glue sets.