Why won't my rechargeable camping lantern work?

I have a gelert rechargeable camping lantern that will not work. It has not been used for some time.I have charged the battery for the stated 15 hours.I have tested the battery and it seems ok. if it is plugged into the mains it will switch on with the remote control or the switch however if unplugged from the mains it still wont turn on. Any suggestions for a techno-phobe please?