1 droopy plant, others fine!

[Kikikickin 10]

Hi,

 

I'm wondering if anyone can help me? I planted 4 ranunculus bulbs a while ago in individual pots. They've been in the same place in my bedroom, in a tray together and have been growing really well.

 

I looked at them yesterday morning..all fine. I looked at them before going to bed and one of them has gone really limp and droopy... I'm really confused! What could have made it do that? Why just the one and not all of them? How do I remedy it?

 

Any help would be great