Yamaha MG10XU Really Low Volume (phones/monitor output)

Hey I use my yamaha mg10xu mixer for my mic and I also use it as an amp for my headphones.
It was working fine, but one day I turned on my pc and was watching a youtube video and the volume was extremely low.
I tried turning up the phones/monitor knob to max, but it was still way to low volume, I could barely hear what was going on.
I’m not sure if this was the right place to post, I’m new to this forum, but if anyone knows how to fix this, then please help me.