Anyway, after the show I felt hungry so I went to the restaurant but it was closed because they were having the inside altered.

inside 在这里是名词，意思是"内部、里面"，指建筑物的内部空间。这个词也可以作形容词或副词使用，但在本句中作为名词，表示餐厅的内部装修部分。