I have been using the register for the past 1 year with no issue. However yesterday, the machine just hang. I tried to pull out the plug and power it up again but it still does not work. How do I reset without losing all the programming data so that this issue will disappear?

It depends what you mean by 'hang'. Do you see anything on the display at all?
Is the backlight glowing in the display?
Does the keyboard respond at all?

First when it happened, the display shows the last number I keyed in. However it didn't respond to the keyboard.
Then I switched it off, unplugged and plugged it back, the same problem happened - there was a display but it didn't respond to the keyboard. The next day, when we tried to switch it on, there wasn't a display at all i.e. appeared to be no power. But why the day before it just froze with the display on but not responding to the keyboard and then now seemed like no power?

I will try to test it using different power outlet.

If you can't get the display to come back on, then you may have a software halt condition. There are various fixes, but you need to see if you can get it to light up again. let me know how you get on.

A technician collected the register last week. This morning I was told there were some parts that need to be replaced and I wasn't please with the price quoted. If the software halt, I wonder what part is going to get replaced. What part could possibly cause the machine not to power up at all? How do I make the root cause that make the software halt be gone?

Oh. you should have got back to me before you sent it in for repair. There were a couple of other things we could have tried.
The machine will sometimes shut down completely and is just waiting for you to do a half clear. There aren't many things which would make the bill expensive, so maybe they are going to replace the main board. You might like to ask them just what parts they are replacing. I can tell you if it should be expensive or not.