Problems with Ceanothus
Generally evergreen Ceanothus do not respond well to pruning but deciduous varieties do so I’m assuming you have the latter. You didn’t say what variety you have and what month you pruned, but the fact that it has flowered this year makes me think that the problem is not pruning related.
I think your problem is almost definitely down to the wet conditions we have had this spring/summer. Even though you have an established tree the root system may have been damaged by excess moisture, this can lead to poor growth and dieback which you are experiencing. This is common with Ceanothus and I have a small variety in a pot at home which has exactly the same symptoms as your tree.
In general you are doing well with your 20 year old tree, Ceanothus tend to live fast and die young; an average lifespan is between 10 and 15 years but give it a chance over the winter and prune off any frost damaged stems to healthy wood and remove dead stems in the Spring. If your tree doesn’t pick up next year and it starts to get a bit sparse it may be time to replace it, but don’t give up on it yet!