Are you able to open the case up? It's frustrating to think that it may be something very simple.
Battery first - as said above and take out any plug-ins that you can confidently remove.
Something is probably 'crow-baring' the input. It just could be a physical object near the supply input.
I'm assuming your battery is flat. Did you cover a change of batteries? Does the unit go when powered up from a spare one? Take care not to knacker another psu or battery!