It sounds as tough the engine has gone into limp mode. This is when a fault exists and that fault is signalled back to the ECU (engine management system) which in turn reduces power output to protect the engine.
Normally you would also get a warning light come up on the dash,however it would appear that you haven't and also have not got any fault codes.

Therefore one would have to assume that a sensor is showing a fault,
You have a accelerator pedal position sensor
An air intake sensor (intake duct)
A Hot mass airflow meter(intake duct)
A crankshaft sensor
A Charge pressure sensor (inlet manifold)
A Injection pump shaft sensor
Not easy top identify what is at fault, would suggest that you check out all the air/vacuum side of the intake for loose/split/damaged pipes. Make sure the air filter is clean. The crankshaft sensor by looking at it, if unsure change it (its not expensive).
After that if still no joy would suggest that it would need to go into a reputable garage for checking out of the sensors.