First you must decide what is wrong with it - a problem statement. What it does that it should not do - or perhaps what it doesn't do that it should do.
Then - when you have a problem statement you develop in your mind what's called - a Most Probable Cause - perhaps other Probable causes too.
Then you test for each - examine to confirm or deny.
Once you have the cause then you can begin to think about the best way to repair it.
With electrical devices the cause is often some sort of interruption in the power supply. If the appliance has a heating element these are often prone to failure.
A householder not confident with repairs to electrical equipment should probably consider having someone with practical experience do the diagnosis and repair.
Good Luck..., January 2016
