why have the leaves gone yellow on my lavitera

There could be a number of reasons. It would help alot if we knew what the plant was.

I assume you mean Lavatera, the woody based sub shrub which grows a lot every year? I don't know how long you've had it, but they're not long lived plants, lasting between 3 to 5 years unless you're lucky - I do know of one that's now 10 years old, so sometimes they carry on, but rare. Did you cut it back in the spring? These should be pruned back hard then, to get new and lush growth. I can't think of any reason why it would be doing this unless you're not watering properly, and I don't know how you're doing that.