one alphabet typed once,repeats continuously on my laptop. Why?

Are you meaning that after you type a letter that it repeats over and over? That can be caused by a couple different things,a stuck key,liquid that had been spilled on the keyboard at some time,a driver problem or even a virus infection or malware. I'd recommend a malware/virus scan to rule those out first.

I dont know what you spilled or what went on your keyboard but it sounds as though you have a sticky button. Meaning: once you push it down, it stays down until you smack it or it comes back up on its own.

How does one fix such a problem? Honestly, I dont know!