Why did he stop responding?

I know this is a common question, but I’m honestly so so confused.

We are friends for 5 years and we talk pretty regularly. In the recent 4 months we talk almost everyday, even about the most random stuff. I sent him a text and he left me on seen, though uncommon of him, I wasn’t caught off guard by this. Then the next day I texted him him Happy Birthday. Once again he left me on read. I would’ve called but I didn’t want to disturb his plans for the day. This was weird, seeing that he was online on social media all day. I understand that people can be busy and forget to reply, but It’s been a week and he hasn’t spoken to me since. He always sends me random memes etc throughout the day so I’m not sure what caused him to go so long without any contact.

My friends say he’s playing some game and they told me not to reach out. Yet part of me wants to. Any advice would be helpful, thanks!
Why did he stop responding?
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