My mobile was turned off since time immemorial and now when I want to

Hi,

Does the phone indicate that the battery is charging when the phone is connected to a charger?

Given that the battery has not been charged for a long time, most probably it has become depleted beyond recovery and will need to be replaced.

Here's a video that shows how to open the phone and to remove/replace the battery. Don't miss it as it occurs within the first 24 seconds of the video ;-)

Search online for EB-GB360CBC to find suppliers of the battery that suit you best.

Once you have a replacement battery, check if it charges OK when installed in the phone and allow it to fully charge before attempting to turn on the phone.