Arum lily, hmm, well there's a deal of confusion about arum lilies - the proper name for an arum lily is Calla, but a lot of people say arum when what they mean is Zantedeschia, common name Calla lily - see the problem? The common name of one is the same as the Latin proper name of another plant. As yours is pink, I'd assume its Zantedeschia - these do much better if stood outside for the summer, and they do like a lot of moisture, preferring quite boggy conditions outside, so it may be that you simply didn't water enough, if you have no sign of disease or infestation present on the leaves, stems or flowers.

Thankyou for your help.You might be right as it is just damp. I will try giving it more water.