membrane switch keypad must be absolutely clean. You may need a new replacement keypad membrane on it if the membrane switches is damaged and unable to pop up back when press.
Each corresponding keys are divided into group of rows and columns. The outer core layer of the keypad are group into rows and the inner core layer are group into columns. Each rows and columns are connected with each other.
The Nokia C3 silicone rubber keypads
The outer core are group from 0 to 7 rows. Each color on the picture indicate where that certain keys are connected to a group of rows.
The Outer core connection line paths ( C3 keypad ways )
The Inner core connection line paths ( C3 keypad ways )
Hardware Repair Tips for keypads: If just only one or two to four keys are not working the problem may lies an open or cut connection to it, trace where that certain keys does not work to the particular group of rows or columns it is being connected. You may apply a simple jumper wire on it as an alternative way. If more than five or more keys are not working you may consider that the problem lies on a damaged EMI-ESD filter chip