1997 Chevrolet Blazer Replaced Alternator On 97 Everything Ran Fine 3 Days S

I replaced the alternator on my 97 blazer everything ran fine for 3 days it's sat for another 3 days in my garage and when I try to start it on the fourth day it wouldn't start it was doing the clicking and then it just stopped altogether there's nothing no sound on the thing battery completely dead is it the alternator bad alternator it was from the junkyard should I get a new 1 to replace that or is there another problem.