It could need a really good soaking, how much water have you been giving it and how often, and, what site/position/orientation is it ?
Plants that are potted need much more water than those in the open ground.

Lavender is very well adapted to prolonged drought, the thick leaves are it's mechanism for preserving water.
One of the biggest causes of Lavender dying out is the overwatering of container grown Lavender or continuous wet soil if planted out. Lavender prefers loose, slightly sandy/gritty poor well drained soil.
They are also relatively short lived shrubs I am assuming your is not very old though?
Do you think you may have over watered fed it?

I have a self seeded lavender into a big pot trying to outdo Encomis bulbs. It is enormous this year. When it droops it gets watered ASAP then left to droop again. I wonder your plant had a visit from the Lavender beetle maybe? Soon it will be trimmed as it is 'going over' and the Encomis is struggling to assert itself. Perhaps try a trim back short of old wood and reposition in a mix of sun and shade. Another lavender in a small pot got chewed by beetle, and it is now late into bloom. Another thought is remove some top soil and top up with a gritty mixture.

I'm wondering if you've been unlucky enough to experience Lavender Shab - if the symptoms are spreading (it usually attacks stem by stem) and the whole plant wilts, then you'll have to destroy them. In the meantime, cut out all stems showing these symptoms, but check the brown areas - you're looking for raised, pinprick sized dark bumps. If you can't see those and the plants aren't starting to wilt all over the place, still cut down the flowering stems and wait and see what happens.

Many thanks for your response. Have cut off all affected stalks, rest of plant seems healthy, no sign of any dark bumps or infestation. It is in a part sun/part shade position. I fear I may have been guilty of overwatering so will cut back on that and see how it goes.
Thankyou again

I would let it dry out totally and then water very sparingly good luck.