It is very likely one of the wires came loose inside the mouse itself. There are 4 black foot pads on the bottom of the mouse. With the batteries removed, pry off the four feet and remove those four screws. You will see two tiny brown wires that correspond to the mouse button.
Examine these two smaller brown wires and ensure they are not loose or have become disconnected from the small computer chip inside. The two smaller brown wires are on the interior of the chip and should be soldered to the board, opposite a larger brown wire and a larger white wire. The two larger wires, brown and white go back into the main cable that connects inside the laptop.
If these four wires appear to be intact, the next step would be to remove the two larger screws in the central blue piece with two larger holes on either side. This will release the mouse from the base. Examine the two solder points where the two brown wires are connected for any breaks. If these look good, there are two more screws that hold this chip in place. remove them and examine the button of the mouse.
Many others have found the actual push button under the mouse fails.
This blog entry explains how he fixed the mouse button:
If you need further help, send a request to:
kjhenseler [at] gmail [dot] com