Your problem may consist in software incompatibility, file deletion, malware or bad/wrong installation. You should go to
Control Panel > System > Device Manager > Mice & Other devices > Right click on your Synaptics device and Uninstall. If the proper drivers are installed the system should reinstall it after you reboot your computer.
Also, may I know what laptop/computer model you have? I should be able to find a new driver or provide the original one for your device.