Lavender plants need really good drainage and my guess is too much water as I see you have the pot sitting inside another container. That could  mean the pot is sitting in drainage water. Lavender plants in pots can also die from lack of water so feel the soil and if is bone dry and if you went for weeks without watering then most likely it suffered from lack of water. The solution is to cut back all the dead dry foliage and remove the smaller pot from inside the larger container. Water when the soil feels dry down one inch. Usually once every 4 to 5 days. If there is any life left you will see new grow sprout from your plant in a week or two. If not, remember you can use those dead lavender leaves for making fragrant gifts. Just harvest the dead leaves and package in small cloth bags for scenting drawers or for craft projects. Lavender likes full sun, drainage and very little fertilizer. Keep growing, Marianne Binetti