Hi,
What's the specific model of your computer?
In generally, It's most likely because of incompatible driver. Please update the corresponding driver from Toshiba Official website for test.
Toshiba Satellite u940-b755
Part no. PSU6VV-01200TAR
Serial no.1D035622k
can u plzzzzzzzzzzzzz tell me which which driver is need to up-date
Visit:
http://www.toshibamea.com/en/Download-Files
After selecting Product Type,Family,Product Series,Model,Short Model No.
Select Windows 8.1 update as OS and select to update Touch Pad Driver,Toshiba Function Key Utility etc.
I shall suggest you to update all other drivers recommended for Windows 8.1.