Cedar tree turning brown

I have a 20-year old cedar that is turning brown in patches from the bottom up.

Let's start with the fact that I'm not a gardener and have minimal knowledge and experience in gardening or tree care.

I'm not even sure this tree is a cedar. I have some good digital photos I could share. Can the photos be posted within a message here?

In any case, my location is near Sonoma, California. The tree has always been healthy and good-looking. There has been no recent digging or activity anywhere near it.

I phoned a tree contractor I've used for trimming in the past, but like so many contractors around here, he doesn't return my calls.

I'm reluctant to call the national company that serves this area. In the past, I've had them shrug their shoulders as to a diagnosis, but recommend a spraying program. I took them up on this twice and lost the trees involved.

I'm open to all advice, including possibly other newsgroups or usenets.

I'd hate to lose this tree and hope someone can help steer me in the right direction.