First, make sure you have the right info (id and/or password) for the WiFi you are trying to log into. Even a small typo can make it not work.
Then...
1. Try and restart the tablet. Sometimes it is that simple.
2. Update the firmware and any apps that need updating on the tablet.
3. If the problem persists, check your time and date on the tablet to make sure it is correct.
4. Should none of the former work...
a. Clear out the WiFi data on the tablet.
b. Reboot the modem/router.
c. Once the modem/router is fully 'up' again, reboot the tablet and re-input the WiFi data.
5. Now, if you are still having the problem at this point, and you are sure it isn't the modem/router that is the problem. Then you may wish to try a...