I recently bought a used 2000 model mazda protégé/323 .it has done 130000 kms. There was a tapping sound coming from the engine head and I was told by my mechanic that the camshaft needs tightening and they proceeded with the job. Now there is no tapping sound but the engine lost power in the process. Could you please help me figure out what could be wrong?

 

There are a few things that can go wrong when the valve clearance is adjusted (which is what caused the tapping noise), and it's hard to diagnose the problem without more info. However, if the power loss is fairly drastic and is more noticeable at high rpm than at low rpm (or vice versa) the most likely cause is incorrect cam timing. The cam timing can be checked by pulling the valve cover and lining up the timing marks; it should only take 10-15 minutes for the shop to check it.

 

It is also possible they have over-shimmed the lifters so that the valves are not completely closing. This would allow blow-by past the exhaust valves or blow back past the intakes. Either way you would have a compression loss. Check the valve timing as carguycw suggested, check the valve clearances, and check the compression. All are easy to do or inexpensive to have done.