I made a project about six months ago, for temperature and humidity control with an Arduino UNO. It was kept functioning continously for almost those six months. I'm not sure about how much care was given to it, I gifted it to a friend to control his snake's terrarium's temperature and humidity. But now, he returned it because it stopped working.
I connect it to the current and it turns on, but it shuts down in two or three seconds. It worked for a long time, so I know my circuit and my program are okay, but what I don't know is if the board was overused or mistreated. ¿What could be failing?
I am a newbie too, but my guess is the voltage regulator is busted.
Maybe 
check the voltage regulator and see if it feels hot as it turns off... they have a thermal shut down. Could be damage if you've consistently drew too many amps, reverse the polarity at VCC pin or various other issues...
Sounds like a short somewhere. What resistance do you get when you measure from Vcc to ground with the circuit un plugged from power? Does the 5V regulator get hot?
I connect it to the current
What is the source of the current?
Does the problem still occur even with nothing at all connected to the Uno?