Hello. I have an electrical outlet that stopped working. There is another outlet on the same line which is working but when I was testing it with my toaster oven, I pulled the plug out and I saw a bright reddish flash from behind the base plate. Both are Gfi outlets behind the kitchen sink. I did try to reset the one not working but with no success. Any ideas or suggestions would be appreciated! Thank you

New GFCI receptacles are about $8... if you know how to safely replace them you may
Find the answer in the back of the outlet box...
Although it is possible the receptacles have simply failed...

What you describe indicates a bad connection in the outlet you plugged the toaster into. That outlet probably feeds the other one.

Looking into my crystal ball, I am going to say that first, these outlets are probably in metal boxes. Second, that the outlets are probably "back wired". Third, the person that installed them left the terminal screws screwed out and that because the boxes are recessed to far into the wall, they left the outlets mounted loosely.
The back wiring probably developed a bad connection. They will do that over a period. When you pull the plug of the toaster out you move the outlet a little bit to one side. An extended terminal screw touched the side of the box and you saw the spark. GFI outlets are relatively large while old metal boxes are sometimes a little small.

Remove the outlet you plugged the toaster into. Put the wires under the screw terminals. If you would like you can then wrap the outlet with a couple wraps of electrical tape, insulating the terminal screws.

Let me know how my crystal ball is is doing.

Bravo! HK...