Could be 2 different problems.
You've checked for hot water going into the matrix by feeling the pipes going through the bulkhead?
The controls are cable operated and if you are getting cold air blowing through but can't get hot air then you've either got a completely blocked matrix or the flap that pushes the air through the matrix on the hot setting isn't working.
When you put the control from cold to hot you should be able to hear the flap moving over and feel the resistance through the control as you put it over
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