1. Let's check out the Reports tab. Click All Folders. Then click the NPSP Fundraising Reports folder to see if there is an out-of-the-box NPSP report that can help
2. The Account LYBUNT (Last Year But Unfortunately Not This Year) report sounds promising. Open the report by clicking the Account LYBUNT title.
3. Double check the report filters by clicking Edit and then Filters. The filters show that you are looking at ALL accounts whose Total Gifts Last Year were greater than 0 (i.e. gave last year), AND whose Total Gifts This Year are less than 0 (i.e. didn't give this year).