A flat battery
THINGS TO LOOK FOR
Try to determine why the battery was flat. Were the lights left on? The glove box light staying on? The boot light staying on? The parker lights left on? Has the engine been cranking slow of late? Has the battery light been coming on?
Depending on the problem. It could be a simple problem of the lights being left on or it could be a problem with the battery not holding the charge. The battery might need to be tested.
You might require a multimeter here. But also you might need to check the terminals.