If you have the written music for a song but do not have those chords on your autoharp, or if the song you are trying to sing is too high or too low for your vocal range, the quickest solution is to transpose that song into another key. You still may not have all the chord bars needed or be unable to hit the notes, but I'm afraid those are separate issues.
At left (or above on smaller screens) is a pdf chart which you can click on to view and/or download to your computer or device. It is a handy reference so you can line up the chords from an unsuitable key and see what to play in one that works better for you. The chart only shows 6 of a possible 12 keys, but those are the ones most likely to correspond to the chord bars you have available.
You can use the chart to adjust the accompanying temperance schedules ("Two Key Diatonic G/D", etc.) to whatever key you require. The conventions employed refer to the chordal scale step as a capital roman numeral, generally referring to chords as capital letters and notes as lower case. Chords are shown as the flat rather than the sharp, and notes as the sharp rather than the flat - in other words, Bb rather than A# for the chord and a# rather than bb for the note - which is in the OS autoharp tradition.
It may be helpful to check out the tuning overview page for a little context of these approaches.