If your SPD or other clipless pedal has stopped “clipping in”, check for small pebbles or sticks caught in both sides of the mechanism. The springs inside the pedal often control both “clips” and so the issue may be on the downward facing rather than the upward facing platform. Remember that you can always rotate the pedal and try clipping in to the other side.
If your pedal has literally fallen apart or off the bike, first check exactly what is broken – has the pedal body come loose from the spindle (axle), or has the whole pedal torn out from the crank arm?
If the pedal body has come loose, it may be possible to repair it. First, remove the end cap from the pedal body (the end furthest from the crank arm) and see if there is a bolt in this area. If so, put the pedal body back on the spindle and tighten this bolt back on to the spindle end.
Some pedals (such as Time ATAC) have the pedal body/spindle attachment at the crank arm side. The same applies here – push the body back on to the spindle and attempt to tighten the threaded part back on to the pedal. You may need some form of thread lock if the threads are worn and loose.
If this doesn’t work, use the spindle alone. You will wear away the bearing surfaces, but the pedal was probably scrap by this point anyway.
If the spindle has pulled out of the crank arm, you can try a thread lock solution to tighten it back in. Are you sure you didn’t force a left hand thread pedal into a right hand thread crank arm? Doing so will strip the threads, but if you find that this was the case, you may be able to remove both pedals and put them on the correct way round.
If the spindle is broken, or the threads won’t hold it on to the crank arm, create a replacement pedal platform. Try threading an Allen wrench through the pedal hole in the crank arm with the smaller arm lashed to the crank (check clearance with the chain stay first). Use this solution more for balance than for power.
Tip: Although the drive side pedal screws on the regular way (righty-tighty), the non-drive side pedal has a reversed thread. To loosen either pedal, hold the wrench with the handle pointing upwards and then turn towards the BACK of the bike.
Tip: If you can’t find flat areas to attach the wrench to the spindle, look for an Allen wrench hole in the end of the pedal spindle that screws into the crank arm.