Is it not working at all? Some Macs have an option to ignore the track pad when a mouse is plugged in. If yours is like this, unplug the mouse or change the settings in System Preferences.
If this is not the problem or only some of the features are not working, check System Preferences to see if features have been disabled.
It might be a problem with the preference files being corrupt. This will happen occasionally. If the first two don't fix the problem, delete the file "com.apple.systempreferences.plist" that you will find in the Library -> Preferences folder in YOUR home folder. Then restart the computer.
Add a new user (System Preferences -> Accounts) and then log in as that user to see if the trackpad work. If the trackpad does not work, then the problem is with your log in ID. The easiest way to fix this might be to make yourself a new log in and then move all your data and files to the new Home Folder. If the trackpad doesn't work with the new log in, then it is a System or hardware problem.
The last resort will be reinstalling the OS. Insert the OS restore disc "Mac OS X Install DVD" that the Macbook came with (or the OS upgrade disc if you have installed a newer OS). Restart the computer and hold the 'C' key during boot to force it to start from the disc. Install the OS with the option to preserve user accounts. If you don't make this choice, the disc will be erased and you will lose all your data - so make 100% certain to read everything on the screen to make sure you select the right option.
After installation (about 45 minutes - 1 hour), let the computer restart and see if the track pad works. If it doesn't, then it is a hardware problem. If the Macbook is under warrantee, contact Apple for free service. If it is not under warrantee, let Apple troubleshoot it and give a quote. After you give the quote, contact the PowerBookGuy in the link below. He does good work. If he beats Apple's price (don't forget the figure in shipping costs - two ways), consider him for the repair. I've used him and am happy with his work.