Customer Service Fail

Today, I had the worst day ever at work. This morning, a customer came in and asked where something was. Instead of helping them out like usual, I just told them to look for it themselves and walked away. I felt so guilty afterward but didn't say anything because I thought they'd already left. Later, my manager came over and said the customer had complained about me not being helpful. Now, every time a new person comes in, I worry they're there to tell on me again. What was I thinking?