1.
used to introduce a different suggestion, after a sentence with if :
I'll go if you're going. If not (= if you are not) I'd rather stay at home.
2.
used after a yes/no question to say what will or should happen if the answer is 'no' :
Are you ready? If not, I'm going without you.
Do you want that cake? If not, I'll have it.
3.
used to suggest that sth may be even larger, more important, etc. than was first stated :
They cost thousands if not millions of pounds to build.