Own dog wants to bite me! Why?

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My parents have a Cocker Spaniel at home named Elmer. He'd been there since the time I was still living with my parents. He had been so sweet, friendly, playful and very lovable. He'd sometimes sleep in my room too. One day, I came home from work, but had a party to attend that night so I rushed in the house, said hi to Elmer then started to get ready for my party. On my way out, I couldn't find Elmer so I started searching the house first. It turns out he was under my dad's computer table, so I called him but he didn't come out. I tried to reach for him so I can pat his head and say goodbye, just like what I usually do. He growled at me and tried to bite me. That was the first time he ever reacted to me that way. And now, everytime I'd go visit my parents, he'd do the same thing (and it looks like he's seriously going to charge at me and bite me) so my mom ends up locking him in his cage first before I could enter my OWN home! Any ideas what made him feel that way towards me?