I was playing for a bit and didn't tune any string, but when I wanted to tune it in at the end of my session my tuner kept saying C♯ instead of D. How do I get my D string back to its original state?
The tuner does not hear what pitch your string is supposed to be at but only what pitch it actually is. If your string is more than a quartertone flat, it is closer to a C♯ than to a D. So your tuner then displays what kind of C♯ it thinks your pitch is. Presumably a somewhat high one (assuming you are not more than a semitone flat). So tune upwards. At some point of time your tuner is going to switch from claiming "too high for a C♯" to "too low for a D". Then go further until the tuner is satisfied with the pitch being D.
The tuner is buggered: get a new tuner.
The string is too loose: tighten the string.