Hi Cliodnha, sorry to hear about your chick.
Are they still in a brooder, or are they off heat out of doors in a run yet?
The first thing that springs to mind if a young chick has diarrhoea is coccidiosis. See this article from Poultrykeeper main site,
http://poultrykeeper.com/digestive-system-problems/coccidiosis and also there's a link to the Chicken Vet website at the bottom of the article, about coccidiosis in backyard chickens. As this progresses you will see blood in her poo, but if this isn't present that's good news.
Diarrhoea in chicks can be caused by a variety of problems, including some viral infections, but is always to be taken seriously. If you have, or can obtain, some Coxoid, available off prescription, you can add this to their drinking water. Otherwise, try to keep the brooder scrupulously clean (I'm sure you're doing this already) and in particular try to keep the bedding dry round the drinker. This is difficult as they're so messy, it may help to put the drinker in on a brick or piece of tile so it's at neck level rather than on the ground, which will help avoid splashes and wet mess around it. Best of all is to fit the sort where a lip protrudes inside the cage and the reservoir is outside, so they just drink from a little cup. Bacteria thrive in warm damp conditions, and this is what the oocysts which cause cocci need to breed. The chicks will peck around in the wet messy parts and pick up the infection. Keep the bedding changed often and poo pick as much as you can.
There's a special disinfectant which kills the oocysts if cocci is diagnosed or suspected, as ordinary Poultryshield etc won't do the job so effectively. I'll go and have a look in my shed in a minute! I forget the name.
She may just have picked up something which will resolve itself, but it would perhaps be best to isolate her, even if only by separating her with a wire mesh from the others. If there is an infection she may have passed it on already, but at least it might keep her poos separate from the rest of the hatch. The hard hearted among us would immediately cull any chick that didn't appear fully healthy, because at that very fast stage of growth any check to development often results in lifelong health problems, and some viruses can re-emerge throughout a birds life and transmit to others in the flock even as an adult bird.