Killed with kindness
Leeks are hardy winter vegetables. They like to grow somewhere cool. Just plant some more, out side in a seed tray will be fine. Or use an out doors seed bed, cover if you must with some glass or plastic, but 20+C on your kitchen widow ledge was to hot for them. The white bits is leek sunburn. It can get very hot through glass at mid day even in winter.
Safer on the cool side than too warm.
Put tomatoes, peppers and chills on the kitchen window sill, they will appreciate it.
