If the actual muster rolls for Co. C, 18th GA Infantry still exist, I believe they are in a National Archives warehouse in Maryland, and are not accessible to researchers. The state archives in Morrow,GA has some regimental muster rolls on microfilm.

If your ancestor was taken captive, there should be a Union POW record of some kind in his Compiled Service Record. If not, he could have been mortally wounded, and died in Union hands before his name & command I.D. were learned by his captors.

Did his widow file for a Confederate widows pension?

A shot in the dark would be to seek out 18th GA casualty lists to learn where he may have been wounded and or captured. There are numerous wartime GA newspapers on microfilm, especially for Macon, Columbus, Augusta, Savannah & scattered issues for Atlanta. Some are available online.

Greg