I just got an email about my Wolf Play account going to expire. I logged into the game, and when I looked at my den all my wolves were gone. Is this a bug, or a normal thing to happen? Do your wolves leave if they don't get enough food/water?

They could have all died due to old age

If you've been offline long enough for the game to send you an email then all of your wolves are probably long dead by now. They age up by a year every Monday and I believe they are at risk of death after they turn 15. Every week after that the risk increases.

So yes. If there are no wolves in your den then they have all passed.

*They age up after 1 game year, which is 12 days. 180 days / 6 months is enough for the wolves to start dying off, so that's most likely what happened to them while you were gone ^^

You can get new ones in the Sanctuary, Auctions, or Trading Post. If you need mush to buy new wolves, you can play games until you have enough, then start exploring to earn more :)