I’ll Walk Alone by Mary Higgins Clark.

I'll Walk Alone: A Novel - Alexandra “Zan” Moreland is an interior designer on the brink of unqualified success.  For the past two years she has put every penny she has earned into her business and finding her son, Matthew, who was kidnapped just after his 3^rd birthday.  Today is her son’s 5^th birthday and several things have happened.  Someone has stolen her credit cards and is charging way more than her limit.  Items for a job that she is bidding on are being delivered, before she has even received notice that she has the job.  And while at dinner with her ex-husband, she learns that there are pictures of the actual kidnapping and the woman in the picture looks like Zan.