是定语从句
So I had practically no social life for the three days (when) my mobile was switched off.
先行词是the three days(三天),关系词是when/或也可以用during which,原因是先行词在从句中的作用：my mobile was switched off (during the three days）,可见在从句中作时间状语的成分,因此关系词要用when或者during which
(when) my mobile was switched off这个从句是用作the three days的定语的,在整个大句子中作定语,是说明“怎么样的”三天
整个句子的分析：
so 、practically 副词作结果状语和方式状语,
I是主语,had是动词谓语（一般过去时）,life是宾语
no、social是个修饰宾语的定语,没有的、社交的
for the three days (when) my mobile was switched off 是介宾结构+一个定语从句用作时间状语
句意：所以我手机关掉的那三天里我事实上没有任何社交活动.