Switching off alarm without code?

We had our electricity supply turned off today which set off alarm then my son pushed some buttons which just made everything worse. Alarm has now been going all day and I can not get it off as we no longer know what the code is and we have not been using our alarm. I really do not want to pay a huge call out charge for an engineer as I'm sure this can be fixed quite easily but I have not found any instruction yet to help me.
Sorry forgot to say it's a Scantronic with 9930 pad