My husband has a 1995 GMC Sierra truck it wasn't cranking easily and so therefore he decided to take the alternator off and replace it with a used alternator. When he tried to crank the truck with the used alternator it would not crank. He then put the alternator on that was previously working and once he got it back on the truck the truck has no power at all no lights on the dashboard it doesn't turn over nothing happening it has no power at all what could be wrong?

Alternator has nothing to do with the truck starting. The starter does though. So, I'm not sure if you are mixing up terms or mixing up swapping parts.

No power no lights? Is the battery the problem?
What is the Voltage reading at the battery?