Thank you very much for your question.
The engine management unit (ECU) has detected a fault and this is why the engine warning light has activated. The car is placed into a safety mode to prevent damage, this is why you feel a drop in performance. The most common causes are faulty camshaft or crankshaft sensors, a faulty lambda (oxygen) sensor, or a faulty coil pack or spark plug. A fault code should be stored on the ecu's memory which will point you in the direction of the fault. The only way to find out what has caused the problem is to take your car to a garage that has the correct diagnostic equipment, which will reveal what the fault code is. Most garages have this equipment nowadays, so I would suggest phoning round for a quote unless you have a regular garage. You do not have to go to a main dealer. There are many reasons why the light could come on, so guessing would not be wise and will almost certainly end up costing you money. You should be ok to drive your Ka short distances unless it develops a missfire, in which case the catalytic converter could be damaged, but I would advise keeping your speed down.
I hope this answers your question.Regards, Gary.
The diagnostic check would normally be around the £40 mark. It's difficult to give a cost for repairs without the results of the check, but it's probably going to be one of the sensors, in which case probably less than £100. I'm sorry that I can't be more specific than that. Thank you very much for accepting my answer.