I had the same problem, this can help any laptop with similar problems.
1- Go to Control Panel / Power Options
2- System settings or Choose what the power buttons does
3- Change settings
4- Uncheck the box "Turn on fast start-up"
5- Save Changes
6- Now you only need to Shut Down and Turn on and it should work.
Why does it happens?
I think it can be overheating, and it locks the processor to that frequency, even if you restart the last information about this issue is not updated, so it stays in safety mode forever. After this change it forces to check some parameters and change again.
thx Guys.