Should I fix my blender while it's plugged in?

My blender doesn't work and I know you aren't supposed to fix things while they're plugged in, but I don't know what's wrong with it and I need to find out. The guy at the store said it was something wrong with the AC adapter and he would sell me a new one, but I don't want to pay $20 for the replacement and I want to learn how to fix things. Is the guy at the store really just trying to make me buy a replacement and spend money, and is it really unsafe to fix while plugged in?