Hi tilly,
Maybe it had an infection or similar, or got into a scrap with a bigger bird, or a lucky escape with a cat?..but if its doing ok now, thats great news. We get loads of birds in our gardens and have been feeding them throughout the snowy weather........we also have two very friendly robins who come right near you and wait whilst we top up the seed containers and then fly about your head waiting to land on them...one day i'll put my hand out....i bet they may just land on my fingers...very sweet :0)