This was my problem some month ago. I have CPU with frequency of 3500 MHz and sometime if you play with tweaking programs to optimize Windows it can hapend to this awful situation. So, here are some tips to solve problem with stucked CPU.
Check Windows power management settings
There is field named "Maximum processor state" which need to be set to 100%
Check BIOS CPU Power Management
Reboot PC to BIOS and search for "Intel adaptive thermal monitor" somewhere in Power management settings. I don't know how exactly this setting works, but ENABLED option cases CPU fixed frequency at 800 MHz.
My worked solution is to DISABLE setting "Intel Adaptive Thermal Monitor"
After PC restart my CPU is back to maximum declared frequency 3504 MHz in my case.
Strange, but it works!