Fire under electric stovetop element
It might be a grease-fire

Take a good look under the heating element - there might be some pooled drippings or other food-gunk. You might need to clean it or just replace the whole unit - as some of the electrical insulation might have burned-off. 

Either way, I wouldn't use ANY of the burners on that stove again until you're SURE what caused it. 

You should be worried anytime there's a fire you didn't intend to make. 

Friendly safety reminder: You do have a working fire-extinguisher in your kitchen right? And your smoke-alarm has fresh batteries\is tested regularly?