There are a number of possibilities. It is a matter of narrowing down and eliminating as many as possible. It either happened from an above ground problem or a below ground problem.

Below ground: What I would do is dig them up and examine the root systems. Dried out? Rotted? Does the soil where it was have a sour stink? Are there massive tree roots in the same area that are competing and robbing the roses of water and nutrients? Root or crown galll? Did the irrigation clog and the roses died for lack of water that went unnoticed? And so on.

Above ground: signs of a disease like downy mildew or canker? Did a neighbor spray an herbicide?

Photos might help.

All of what hoov said and also look for moles or voles or gophers attacking the roots from below ground.

My vole, mole, etc., damage has been more of a one day just found them dead lying on ground. Agree with hoovb..as usual.

Thanks, this is very helpful. I'll pass it along!