It could be in the relative minor key. Every Major key has a relative minor key, which contains the same notes, and the same chords as the Major; it simply starts on a different note. For example, in the Key of C Major, the relative minor is Am. So if the piece looks like it’s in C, but starts and/or finishes on A, it’s probably in A minor. The way to find the relative minor key from each Major key is to simply count up 6 notes from the root. For example in C Major: C=1, D=2, E=3, F=4, G=5, A=6 (And this is the minor key). Note how it’s different from the A Major key.