remove the 2 screws to take the bottom plate off
remove the 2 screws that hold the battery chamber
this is the negative connector that's loosed up, glue it back to the battery chamber with super glue to get the result on the next picture
then put everything back together after the glue has dried
and it's fixed and ready to go
-as suggested on the discussions, I also added a piece of small foam between the battery chamber and the body wall to secure it
-you may want to clean the contacts first it see if that's your solution
-it is also easy to put the battery in the wrong direction so remember to double check
-also check the solder point of the battery wire, if it's corroded, solder in a new one to keep the camera working
Some references and more solutions if this doesn't help:
Discussion on flickr about the problem
Discussion on photo.net about the problem (I followed Winfried Buechsenschuetz's instructions)
Camerapedia about Canonet 28