Awkwardness
Explanation
The Joke
A woman is panicking because she sent a sexy email to the wrong address -- specifically to her ex. She considers handling it maturely ("Well, I could just be an adult about this"), but in the next panel the alternative option is presented: she visits a cryonics facility and asks them to freeze her body until "things stop being awkward." The technician asks, "How about just the head?" -- implying this awkwardness may literally never end, so a full-body freeze would be wasteful.
The Humor
The comic escalates a relatable social embarrassment (sending an intimate message to the wrong person) to an absurd extreme. Rather than simply dealing with the awkwardness like an adult, the character would rather be cryogenically frozen -- essentially choosing to skip ahead in time past the discomfort. The technician's suggestion to freeze "just the head" is a dual joke: it references the real practice in cryonics of head-only preservation (neuropreservation), and it implies the awkwardness will last so long that full-body preservation is impractical. The humor comes from the exaggerated lengths people will go to in order to avoid social discomfort.
References
Cryonics is the practice of preserving human bodies (or just heads) at very low temperatures with the hope of future revival. Organizations like Alcor Life Extension Foundation offer both full-body and "neuro" (head-only) preservation, the latter being cheaper. The technician's offer of "just the head" is a nod to this real distinction.