Thank you for all answers
[THE LONG STORY]
I've been using official ROM just like usual. Then the phone [GT-i9500] suddenly restarts itself and stuck in a bootloop.
I've tried entering both recovery / download mode [vol+home+pwr with the sim and sd card removed] - nothing worked. More specifically, after I press 'continue' to the download mode there comes up the GT-i9500 splash screen. I doubt that my keys are not working tho.
What's funny is that, by the way home, I took the phone to the Samsung service center. While pretending to be just another dumb user, they plugged in a USB JIG and the download screen came up like normal. And they tell me that the mobo is broken and want to take my money for the fix, bleh. Unquestionably, I declined that.