Is my lawn dead?

I assume you've only just cut it and it looks absolutely dreadful - brown, yellow and deadish. It will recover, but you need to cut it regularly and it wouldn't hurt to apply Lawn Food, or WeednFeed if you've got weeds, once it starts growing a bit.

Bamboo is right, its where nothing is getting to the ground cos of the length

will be fine once its had a good feed and a decent haircut every so often

x x x

Yep. I neglected mine once and never cut it. No sun got through the length, but it did recover and quite quickly too with lots of sun and lots of water. Like Bamboo says, feed it too.