Chook Died

Very Sad
one of my 2 chooks just suddenly died?
it was fine in the morning, fed them as usual in evening its laying on its side barley moving, little latter its dead.
The remaining chook is very distressed and seams to be missing his buddy.
Any advice on the cause.
Is it ok for one single chicken to be on its own!
could it have started its first egg which got stuck!
oh well thats nature but still dosent feel good!