Platy help?

Hi,
I was wondering if anyone could help me.
I was given a small tank that has two platys in (I'm brand new to fish, so I apologise for the stupid questions) and I've had no trouble for weeks until tonight. I checked on them before bed to find the one on its side and it almost looks like there is a cut on the stomach. It still swims but it stops.
So am I losing the plot that it looks like there is a cut on it's belly? Or is it something else? Is there any way to save the fish?
I really do apologise for the stupid questions but I never thought I would be so attached and I hate the idea that it might be suffering.

Thank you for any help,
Tracey

Unfortunately it's died but can anyone give me an insight into why?

Based on the swelling I would guess a parasite of some kind. Not really Easy to tell tho from just seeing a picture without behavior etc. Sorry for your loss!