I have an A11 that stopped, after being left with a battery in it for 11 years!!
For the A11 remove the ruber gasket then remove the 3 screws on the the side farthest away from the bulb. That side will then lift and there are 2 catches on the other side.
On mine the corroded battery in the unit has corroded the solder attachment at the bottom end of the battery holder and the negative cable has come adrift. Looks like an easy solder job once I've cleaned the corrosion off.
My Olympus A11 stopped working too. Here's what I did to fix it.
1. Remove rubber gasket on the side that attaches to the camera. The gasket runs from the top to the bottom of the A11.
2. Then remove the 3 screws on the the side farthest away from the bulb. That side will then lift and there are 2 catches on the other side.
3. On mine the blue wire from the negative terminal of the batter came loose from the circuit board.
4. Since that broken wire seemed too short, I soldered a new longer wire from the negative terminal to the circuit board.
5. Now it works like a champ.