Sad/frustrating library moment

I finally had one of those reference moments where I couldn't really help a patron. She wanted to get online to find contact information for a professor who she was supposed to meet so she could take a make up test. I told her I couldn't help her and just as I was telling her she would need to go to the reference desk to get a log in she walked away really upset. I felt frustrated that I couldn't help her, and it bugged me every time I saw her. She later asked to use the phone at our desk, which we're not supposed to let patrons use, but I still felt bad that I hadn't been able to help her before. So I let her use it, and she didn't use it for very long. But she was pissed off at me, which quickly got rid of my guilty feeling. You'd think if you had to make up something you would come prepared, such as writing down the cell number of your professor in case you couldn't find them, or at least borrowing a cell phone if you didn't have one to contact them. If she hadn't acted so snarky and rude I wouldn't be feeling this way. I now know the frustration of not being able to help a patron. Not something I want to go through again, but I probably will.