Advice for my parents

My parents have been married for 36 years in January. My mom recently told me that things aren't good between them. She said he seems to not care about anything, complains about a lot of stuff, gets mad when she tries to talk to him. She said she still loves him but said it feels like they are just friends, that things have gone cold. She said she wants to make it work but doesn't know how to fix it. Any advice for them?