cucumbers

hi,
im growing 2 cucumbers in 1 container out doors , they have grown to about 2ft & are looking very healthy, but at the base of the stem slighty above the soil the stem has thinned & on 1 side & has turned white, the affected part is dry to touch. there is also a bend at that point which i have supported. the affected part is about 4'' long.
this is the first time i have gorwn veg so would appreciate any help.
many thanks
paul patrick