My rabbit just lost a claw. Should I be concerned or not?
Rabbits lose toe nails when they are torn off. It is not normal, but neither is it rare. The two biggest contributors to this painful occurrence are untrimmed nails and places for the nails to get stuck (wire bottom cages, gaps in ramps, etc).
When this happens, check the rabbit to find the extent of the injury. If most of the nail is still there and it is just broken, immediate treatment should be the use of 'septic powder', which should be in your first aid kit.
On examination you may also find that the nail is completely gone and/or that the toe is broken or other substantial injury. In this case 'septic powder' alone is not sufficient. A trip to the vet is required, and it is just as important to do so as if you just ripped off the tip of your finger and/or broke and mangled it. Treating for potential infection and pain management may be indicated.
In all cases, prevention of further injury is critical. Rabbits can and do get broken legs from the same things that lead to broken toes. As prey animals their instinct is to try to escape when they feel injured and/or caught. Where you or I might realize a finger is stuck and work to loosen it, they will often bolt causing further injury. They will also try to mask any injury so as to appear healthy and decrease their chance of becoming someone's dinner.
Even with trimmed nails you need to ensure that there are no places for toes to get caught. Wire bottom cages are particularly problematic for long term housing. If you can identify specific areas where a toe or foot can get caught repair them. If you have an area that you are not sure how to fix, post a picture (as a new question) and ask for help correcting it. We want to help you make your pet safe and happy.