Thank you for your question.
The engine management unit (ECU) has detected a fault and placed the engine into failsafe mode to prevent any potential damage being caused. You may also have noticed a drop in performance. Unfortunately, in order to find out what the fault is, you will need to take the car to have a diagnostic check carried out which will reveal a fault code. From there you will be able to carry out any repairs that are needed and then have the ECU reset to remove the stored code and remove the error message.
I hope this answers your question.
No, most good garages should be able to carry out this check. I would suggest phoning a few to for quotes as prices may vary.
Their are many possible causes. Anything from a temperature sensor to catalitic converter. Unfortunatly, without a diagnostic check it's impossible to know.