Clematis need a lot of moisture and yellowing then browning out at the bottom working up is fairly typical.

I would cut it back hard and make sure it is well watered. Don't rely on the rain, I can't do that even in Oregon

For what it's worth, a knowledgeable employee at my nearest locally-owned nursery told me she thought her favorite clematis died, and it surprised her by re-sprouting TWO years later. (That being said, I have obelisks still in place where two The Presidents died last year, and they have not magically reappeared. . . )

I usually mulch the area around my clematis well (though not right against the stems) to help keep moisture even. A couple of times a week you might stick your finger down as far as possible into the soil in the area you dug out. Sometimes the moisture level in a relatively new hole may be different than what it appears or what the surrounding area is like, either drier or wetter, so checking before watering is helpful.

A few questions:

Where are you generally? - plants behave differently in different parts of the country

What is your soil like - sandy, clay or loamy?

and what is on the other side of the fence? - i.e. might your neighbor or a road maintenance crew have been spraying something like weed killer?

For what it's worth, IME Jackmanii is a fairly robust clematis and should pull through even though it may not look too ornamental right now. My mom had one that a landscaper planted, in clay soil with no attention whatsoever other than a pruning if I happened to be in town at a good time for that, and it survived and bloomed every year.

My newly planted jackmanii is doing the same thing this year. Planted in clay with some compost. Did fine for awhile but eventually started getting a black mottling (not spots really, more like a "rash") and the leaves turned yellow and dropped off. I moved it to a pot for the rest of the season as I think the area it was in might have retained a bit too much water. Hopefully it survives... I'll have to find a new spot for it that's a bit higher. Kinda bummed... I had great success with my 'Sweet Summer Love' last year and this year, so I thought jackmanii would be pretty easy :/

Remove the mulch from the base of the plant (about three inches around) for two reasons;

1.) bark and wood mulches can leech nutrients from the soil and leave plants weak (all you need for mulch is a nice compost)

2.) new baby vine shoots cannot make their way through mulch

Try to deep water using the drip method once a week and find a way to cover the base of the vine from the heat of the sun (I attached a link to the right). Cute option to cover vine base.

Good Luck!

Bark and other wood based mulches do NOT rob the soil of nutrients!! Wood products - when mixed into the soil - can tie up some nutrients as they decompose but when used as a mulch they cause no issues!! This is a garden myth that has no basis in reality :-)) And mulch is no deterrent to new shoots from the root crown emerging either.

Clematis roots need no projection from the sun - they are located quite deeply underground if planted correctly. What they need is a consistently moist root run and shading with another low growing, shallowly rooted plant like a perennial or groundcover can accomplish that.......as can a good layer of mulch (really any kind you choose, although I'd lean to an organic/natural product as opposed to a non-organic mulch like rocks, etc.).

I recently removed all mulch from my garden and replaced with a couple inches of compost and my plants have never thrived more. Try what works for your plants and form a conclusion for yourself.

I mulch pretty much all gardens except those planted with dry-land plants with wood shavings, chipped wood, or bark mulch. Everything does just fine - as GG48 said, the wood doesn't use nitrogen except perhaps right at the interface between the soil and the mulch, not where the plants' roots are.

As jnavarro said compost is of benefit to plants, but that doesn't mean wood-based mulches aren't - they help maintain even moisture and temperatures, and over time will break down to release nutrients and improve the soil's tilth. It just takes longer than with compost.

My clematis emerge through mulch without issues.