Use styptic powder or even flour to stop the bleeding.
Did this happen while you were cutting the nail? If so, then the trimming was too close to the quick (blood vessel inside the nail). When cutting, make sure you look at the nail and find where the pink area is. If you see pink inside the nail, that is the quick - Don't cut too close, especially if your rabbit is not used to getting its nails trimmed. If your rabbit has dark nails like mine, then you will need to use a small light to look through the nail to find the quick. I personally use a small pen with the light at the end of it.