I went back to check the pilot light window in a different angle, this time I see the blue light with my eyes. After some adjustment to the knob, the heater now is working. Hot water is back! However I noticed it acts bit differently from before. I used to see a tiny indicator light blinking all the time but now no longer exists. Also it used to make noise when hot water is running, but now dead quiet! I do hope it remains functional. I would like to thank Randy and Larry for your time to help me out. Will let you know how it goes. Many thanks.