First thing to try: see If you can't change it back if you boot into Safe Mode.
If not it's time to go back to factory conditions from the recovery disks you made, the recovery partition or the refresh or reset options offered after you boot from your Windows 8 install or recovery disks.
Since all of that is rather depend on the make and model of your laptop, I can't be more specific.
Kees
OS:windows 8.1
I made the dumbest thing i've ever did.From MSconfig i changed maximum memory allowed to 256mb....And now when windows boot i got "BAD_SYSTEM_CONFIG_info" error.Please help me.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic