I got caught playing a video game at work. Will I get fired?

I was done with work and was having a little break. There’s an event in a game that I was playing at home where the winners would get a good in-game item. I got excited since I was one of those winners and decided to install the game and login. I was very discreet, the game is on window mode and I hid it when someone passes by my cube. An hour after doing that, I got caught and received a private message from one of my managers asking why am I playing games on work hours. I explained to my manager that I have finished all my work at the moment and is just checking on a game. I apologized and said that it will not happen again.

After a few days I got a ‘Noted to Explain’ email from HR which they gave me 5 days to explain. The email also says it may lead to corrective actions up to and/or including dismissal from company.

Will I get fired? What’s the best way to tell HR that it’s only a one time occurrence and that it will not happen again. Any other advise that can get me out of this situation? Serious answers only please, please.