My truck is dead every once in a while. I got a new battery thinking it had a dead cell but still have the same problem. I do not think it it’s the alternator.
Now I can not even jump it.
More info,what happens when you turn key on. What happens when you try to start?
Maybe check under coating of hot wire and all connections for crud.
Make,model,engine size on all posts!
Its time to just pull and clean all of the connections from the battery - both ends of both cables. If that does no good then get under the insulation to check the actual cable as hd72mm said.
~ that’s assuming that when you say “dead” it means that you turn the key and little to nothing happens.