@JBrough If your key is full have you tried putting it into the meter so that the meter can read the credit you have bought. Paying for the card in the shop is only part of the process, you then must put the card in the meter.
Most of the time when a pay as you go meter screen goes blank, it has went into "safe mode", your electricity supply will not cut off in this mode, BUT, if you do not keep topping up your key in the usual manner (KEEP ALL RECEIPTS) you will build up a debt that will be based on and worked out via your past electricity usage.
A meter in safe mode will no longer be fixed, instead Scottish Power will tell you that they need to fit a SMART METER to your property. If you want to avoid having a smart meter fitted, tell them that the bill payer is severely autistic and cannot deal with that kind of change and a replacement pay as you go key meter should then provided.