Box Select Doesn't Work!

Box select use to work in the dope sheet editor, now all of a sudden it doesn’t work…

What did you change ?

Are you using the default blender shortcuts
Does box select work elsewhere ?
Check the shortcuts in the USer Preferences / Input for the dopesheet

Restart blender

I don’t know what happened, it suddenly came back. But it changed after I changed the extrapolation mode to linear. Btw how do I change that back to smooth curves?