The two most likely candidates:
1) Plugged heater core (if loss of heat came on slowly over several months)
2) Failed solenoid valve (if loss of heat came on suddenly)

Fortunately these two components are located very close to each other and
require the same access.
Either job can be done in a Saturday morning.
Get yourself a Bentley manual for detailed instructions.

1) You have a low coolant level

2) the heater control valve is kaput

3) the control circuitry is dead