I sat on my 24 x 6 1/2 ruler when I was at the LQS for a class - the crack was so loud everyone heard it and of course it was embarassing to say the least. As it's my favourite ruler, I had to figure out how to repair it - here is what I did:
Read all the way through first to make sure you have everything you need ready at hand -
I found a place I could brace one end of the ruler tight up against, while laying it flat on the surface ( by co-incidence, my desk has this slight ledge along one side). I put parchment paper underneath where the join would be. I test fitted the edges a few times to make sure I could line them up cleanly and quickly.
I used Gorilla Glue (like super glue or Krazy glue) to baste a small thread of glue along the joining edge of one piece.
Then I lined up the edges exactly, pressing down from the top as well as from the side that was not braced.(Because the other side was braced/wedged, this gave me something to push against).
I held it in place like that for a minute or two, then I took a spare, 4 inch square ruler, put glue around the square about 1 inch in from all the edges. I put it in place on top of the join, lining up it's marking lines with the ruler I was repairing, more or less centering it. I held that in place for a minute or two, then I placed a very heavy weight on top, another heavy weight pushing from the unbraced side and left it overnight. I am not sure if the weights are required but I didn't want any shifting about.
The next morning, I took off the weights, peeled off the parchment paper and used a paring knife to gently scrape away any excess paper or glue.
This ruler is now pretty much good as new, I can see my lines, I can line up on my fabric and I can cut along it's edges, even with the join as it is barely noticeable. I am quite happy with this repair - all my rulers are the same make/manufacturer with the same pattern of cutting lines and I can't buy them anymore, so I really wanted a workable repair.
If you need more info, or pictures, please PM me.
Good luck,
GEMRM