Wire brush the back of the hold and the area where it came off, remove any old sika/resin that wants to come off (if it's well stuck on then it's probably ok to leave). Stick back on with some sort of two part sika/resin, the strongest being pure epoxy, the best for damp rock being epoxy-acrylate. I use the same gun applied stuff I'd use for sticking in resin bolts. Don't forget to discard the first few squirts from the mixer nozzle. Stick a piece of gaffer tape over the rock to hold it in place while the resin cures.
If the hold you kicked off is the 'sugar-cube' on In Brine then ignore everything I've said.
