。
Keiko buried her face in the pillow and cried.
Literal
Keiko [topic-は] pillow [in-に] face [object-を] bury cried.
うずめる appears here in its literal physical sense of 'to bury X (in Y),' distinct from the figurative use of filling a venue or space. Pillow-face-burying is the canonical context where this verb reliably shows up. The particle 枕に marks the pillow as the destination/container, and 顔を marks the face as the direct object being placed there. A tiny two-clause sentence — bury and cry — chained with the written-style connective form うずめて.