Hi @leighacox
Try the following and check if it resolves the problem.
Remove the batteries and the ''back up battery'' from the calculator and then ''press and hold'' the ON button for a full 20 seconds and then release it.
''Note:'' Remember the way that the back up battery was installed (+ve on top as seen on the battery itself) so that you reinsert it the correct way.
Reverse the procedure to reassemble the calculator and check if it turns on and works OK.
Here's the ifixit [guide|117983] guide that may help. You don't have to go past Step.5