heat won't come on, only the ac with a heat pump

by John
(Oak Island nc)

Trane model no. twe024p13fbo for the air handler and I can't find a no. on the heat pump downstairs.

When I tried to put the heat on the other night we got no heat. The ac works fine with the fan coming on and the compressor down stairs also working fine with hot air blowing out but when I change the thermostat to heat absolutely nothing happens. Also I found a capacitor on top of the air handler and it looks new. I had a new electrical box put on down stairs in june when the breaker kept tripping and maybe the repairman left this when he was here. This is a rental property. Thanks for your help.