Olympus x840 screen displays 'empty batteries' even tho fully charged batteries installed

Always be sure the batteries are charged.
Insert the batteries correct plus to plus and minus to minus. Only one battery the wrong way around and the camera won't work. Check contacts of the batteries and in the battery compartment of your camera. Dirty or oxidised contacts can be cleaned with a dry cloth. Never use sharp tools to clean contacts or anything in or on your camera.
Always charge the batteries before using it for the first time, or if you have not used it for a long period. 3 months is a very long period.
Also check if the battery door is closed correct. Most cameras do have a switch connected to the battery door. If the switch does not work correct, it could be you can't switch on your camera.
Do not use alkaline batteries unless it is absolutely necessary. In most cases, alkaline batteries have a shorter service life than NiMH. Alkaline battery performance is limited, especially at low temperatures. The use of NiMH batteries is recommended.
Never use manganese (Zinc-Carbon) batteries in your camera. They can't deliver enough power and can start leaking, causing damage to your camera.