1. Clogged heater core. Common
2. Problem with the blend door, which is not allowing heat into the cab. Happens, but less common.
Verify blend door operation. If that is okay, you have a clogged heater core. I always try flushing them both ways with garden hose pressure to start with. If that is not successful, I have had luck with the CLR method. Here is a link to a video to watch on that.
I try the CLR method as a last ditch effort as replacing the heater core is a pain in the mule.