Last time I used 4 days ago, it worked fine. Today the fob key didn’t work so I used the key. But I couldn’t start the engine and all the other functions don’t work such as lamp, horn, dashboard. Is it only the battery or anything else?

 

Do any lights inside the car come on? If it's just totally dead then it's probably the battery (or a battery connection maybe came loose?). Battery could die of old age, an electrical short, a power draw left on for days (light or maybe charging something if the 12V port stays active without the ignition), or a failure in the charging system catching up to you. Could also be that the security chip-thingy in the key fob has failed and the car won't start without it, but I'm not sure what other systems that would disable. If you have a second fob, try with that. Most car parts shops will test your battery for free. That's where I'd start. Just remove it and take it in. Good luck!

 

It is most likely the battery you could try a jump start.
If that works get the battery load tested and replaced if needed.
If you haven't replaced your FOB batteries in past few years, I would do that as well.
Most car batteries will only last 5 years tops and many start declining after only 2 years.

 

It was the battery. I bought a new battery and it worked. Apparently I forgot to turn off one of the lamps inside.

 

Glad you're up and running without too much of an issue.
That's why I never use interior lighting that doesn't automatically shut-off, I can't count on remembering to manually do so.
I actually keep a flashlight in the console.