Lock & Key (Lock & Key 1) by Cat Porter

Once Grace and Lock had nothing. Now, they both have so damn much to lose.

One fiery night with a beautiful woman makes Lock feel alive again, but he doesn't realize it's her, his obsession from the past. His mentor’s widow.
When her biker husband was murdered sixteen years ago, Grace Quillen took off from her small hometown forever. But when tragedy strikes again, forty-two year old Grace must return to care for her sister and little nephew. One night she indulges in explosive heat with Lock, an attractive stranger she meets at a bar, but the morning after, she's shaken by her fateful mistake.