Does anyone know about renewal of expired Green Cards for USA?

Hi there,
my girlfriend marrie an American – got a Green Card but it expired and she didn’t go back for 3 years, however she has been back for forward since then – she thought that her Green card was terminated so didn’t bother getting it renewed.
However, because you have/had one doesn’t that mean you always will be able to get it again – eg : if she were to reapply?

I hope this makes sense?
Thanks