Some laptops have a quiet mode feature. It tries to reduce how much heat your computer produces and disables fans. On some laptopts it's controlled using the keyboard. It's easy to accidentally enable if it's one of the function keys alongside volume control, screen brightness, keyboard backlight, etc.
If it's not controlled by the keyboard then it's possible that it's a BIOS setting.
Your CPU is capable of running at variable clock speeds. The frequency is usually some multiple of 400 MHz or 800 MHz on consumer Intel machines. This mode locks the clock speed to its minimum value. It is supposed to make the computer draw less current and thus run cooler. (Of course with the fans disabled it won't stay very cool.)
I'm not sure if the setting can be disabled by software. From my experience with this issue on Windows 7/8 the only way I could disable it was using the keyboard.
You should also restore other power settings to their default values after toggling "quiet" or "fan-less" mode. There are power settings in Windows that will similarly throttle CPU speed to reduce power consumption. (Such as the "system cooling policy", which can be set to either active cooling (fans) or passive cooling (throttling). I think in active cooling mode both fans and throttling are used but throttling is only enabled if your computer runs hot even with fans active.)
You can see similar throttling (normally temporary) if your fans are broken or don't work well. Assuming the clock speed is stuck at one frequency (even immediately after booting when the computer is still cold) and that the CPU temperature sensors seem to work, then it's probably because quiet mode is enabled.