There may be a note at 335 Hz, but by definition it isn't A. The point, though, is that some tones blend pleasingly to the human ear, while others clash gratingly. If you sing a tone that clashes gratingly, it's a sure bet you're off key. It may be a little off or a lot off from the desired frequency (440 Hz is a frequency), but if it's off it will sound dissonant to most people.

Many of us have vocal ranges that can't follow a song in the key it's commonly performed in. There's no simple solution. You can harmonize (sometimes), change your vocal range, or get the song played in a key appropriate for your range. How easy or possible these options are varies with different folk's circumstances.

If you really get to where the note you want is not on the keyboard of a properly tuned piano, I don't know what to tell you, other than it could be a case of "misrerembering" the specific note.