lost all power

I have a 1973 mgb. I got it last fall and only drove it a few times. yesterday when I tried to start it the battery appeared to be a little low as the starter cranked a little slow. it cranked a few seconds and then I heard a pop or click sound. I now have no power to anything. is there a main fuse or circuit breaker somewhere on the car. the car is original with only 17000 miles on it. any help would be appreciated thanks Denny