My guitar sounds flat?

I bought a guitar off my friend that was virtually brand new. I tuned
it and It was fine it stayed tuned for a few days I retuned it with my
electric tuner but It sounds sooo bad. It is flat :/ And sounds
diabolical, The tuner says it is in tune and everything but when I
play you can tell its not :/ Any ideas of why this is? And what should
I do? Thanks :)
No its an accoustic guitar :(