My daughter tipped over our Canon 40D camera (on our very sturdy Manfrotto tripod) while getting freaked out by a thunderstorm. The lens took a header and "fell apart". This lens in our favorite lens. Unfortunately, it is inexpensive (<$100), so there was probably no point in having it repaired by a service center. My only choice was to buy a new one -or fix it myself.
I started to look for new lenses. I considered the Canon EF f1.4 from Amazon, but it was just a bit to pricey at $349 from Amazon. A new Canon EF 50mm f1.8 from Amazon had gone up since I bought it, but was still a reasonable $100. Well I figured those were my best two new alternatives, unless I fixed it.
Fixing the 50mm f 1.8 lens:
I did try just "putting the lens back together" -that wouldn't work. Nothing just snapped into the right place.
I found some great advice from this Flicker post about a broken 50 mm 1.8. I decided I had nothing to loose, so I followed the 50mm f1.8 dissassembly instructions mentioned in this post. The guy who put them together did a great job (Yosuke Bando). So much so I moved a copy of the pdf to my server so I'd always have it.
Following the instructions word by word I was able to properly disassemble it -then reassemble it. It only takes about 15 minutes -once you know what your doing. The first time it took awhile to figure out how Yosuke named all the parts of the lens. I had to do it about 3 times -remember the follow the instructions part. I'd recommend printing out and reading the instructions.
I did need to do some straightening on the steel ring that holds it together the lens together. It got bent when the lens hit the ground. To straighten it -I traced a circle of the lens on a piece of paper and then used linesman pliers to gently squeeze the ring back into a perfect circle to match the circle on the paper.
If your favorite cheap canon 50mm prime lens gets dropped and falls apart -hopefully you'll be able to use my experience to put it back together again. Happy picture shooting. PS -these are great lenses for the price.