Cross of Fire by Lester del Rey

Karl Hahrhöffer wakes up soaked and disoriented in a storm, with no memory of how he got there. As he stumbles back to his old, decrepit home in Altdorf, he uncovers a series of strange and unsettling events. Villagers fear him, his house is in ruins, and his own reflection is foreign to him.

With only snippets of memory and a mysterious diary to guide him, Karl pieces together a chilling truth: he was once a vampire, turned human again by a lightning cross. Now, he must confront his undead past to find peace.