ActiveX CommandButton Stopped Wroking

This is a small portion of a large workbook I have received from someone in my company.
I am asked to write some summary reports etc.

Anyway, the original file has these comandbuttons that seem to stop working at times.
The attached file has two worksheets and the button on "C1" worksheet should take you to "Input" sheet but it doesn't do anything. And the button on the "Input" sheet at Cell address "U7" is supposed to take you to Sheet "C1".

I am not too familiar with ActiveX buttons, so I would appreciate any help I can get.

Thanks,
modytrane.