wetness is the most likely cause. Is /was it an evergreen one? double check the shrub by doing the bark scratch test. using your thumb scrape the bark back, if it is brown that bit is dead. if green underneath its alive. work your way back from the tips at say 6" gaps and see how much is dead.

Ceonothus are short lived and die when you stare at them a bit hard. Mine died from dry roots one summer, no pleasing some plants

I replaced it with a pyracantha FWIW

Chris, my now departed Ceanothus developed a nasty brown sticky sap like substance along the trunk and I was advised to remove the whole shrub. Shame, as it has bloomed so well the year before.

that sounds like a bacterial infection, possibly a canker. One of mine did that too.

Sbg, I think you advised me about the problem when I posted a question about it. It had flowered beautifully for some years.

I've lost four in recent years, two of which were huge and very healthy looking one day, then started to turn brown, they died completely within a week, tried new ones and planted in completely different parts of the garden only to have the same thing happen, I've given up on them now...

Thanks to all of you for your replies to my question. It definitely sounds as if Ceonothus are extremely vulnerable so I shall not be replacing ours! It was a wedding anniversary present and that is the only reason I am
sorry we lost it.

Most are native to the west coast of North America, so think mildish wet winters, dry, warm to hot summers, and either old sand and gravel beach deposits, or sand dunes, for soil. Frozen soil or humid heat are very stressful to them.

Our soil is all wrong by the sounds of it Tug. It is heavy clay in places. Thank you for the information you gave me.