No idea, you have to provide a bit more info. There could be a million possible reasons for this problem. You didn’t even mention what DE you’re running.
Are you using an Nvidia card? Do you have proprietary drivers installed?
What happened roughly 3 days ago that might have caused this?
You’ll have to provide a bit more info if you want help. Start by posting logs and hardware info (inxi -Fxxxza).
Try not to do that, this can cause corruption of the filesystem and data loss. You should consider enabling Magic SysRq button to safely reset your computer in case it freezes.