Are they the ones that are sold as 'indoor' plants? If so, they're not due to be in flower for a month yet, so will have been brought on for the market. They tend to be over fertilised, and soft, so are prone to sickness when conditions aren't exactly what they've had. Too dry, too wet, they immediately react. Possible they were overwatered and rotted at the base. I would stand them in a shady spot, reducing the stress on them, and let them recover. Remove any faded flowerheads and any dying/browning growth. Hope.

Thanks for that - will try your suggestion and keep fingers crossed!

If you bought them from a good garden centre or nursery, you can usually take them back for a refund.Were they healthy when you bought them?