Some comments and ideas from Google "
AM radio not working FM works fine"
If the antenna was not connected, the FM would still continue to pull in stations and function to a lesser degree and the AM would not pull in any stations at all.
"I've noticed that after I wash my car or if it rains my AM radio stops working. After a couple of dry days it comes back. I'm guessing that water is getting on the AM antenna connection".
"Just the AM quits, the FM works fine. wiggling the antenna seems to bring back the reception so I guess the problem is at that end".