PLEASE NOTE: It is not advisable to do a reset unless you have exhausted all other options, as doing so will cause the phone to lose any and all data you have saved to it. This includes address book, apps, music, photos, etc. So if you can access the info on the phone, and the SD card, you might want to backup any and all of your personal info before performing this.
That said, you can try the following...
To perform a master reset from the settings menu, follow these steps:
1. Back up all necessary data, on both the phone itself and the SD card. (Yes, I am saying this again, it is worth it.)
2. Now go into your "Settings" and scroll down to "Personal".
3. Now tap "Backup & reset".
4. Under "personal Data" tap "Factory data reset".
5. Enter your pattern, PIN, or password if prompted.
6. Now here you also have the option to erase the SD card. Should you do so, it will remove all things saved there, rather than saved on the phone. If you chose to erase it then select "Erase SD card", then tap "Reset phone".
If for some reason you cannot get into the phone to do the above, you can try this way...
Master reset with hardware keys
To perform a master reset with the hardware keys, follow these steps:
1. Back up all your data. (Yes, I said it again.)
2. Turn off the phone.
3. Press and hold the "Volume Down" and "Power key" at the same time.
4. When the phone vibrates, release the "Power key" ONLY.
5. When the Android recovery screen appears, release the "Volume down key".
6. Press the "Volume down key" until "wipe data/factory reset" is selected, then press the "Power key".
7. Press the "Volume down key" until "Yes - erase all user data" is selected, then press the "Power key".
8. Press the "Volume down key" until "reboot system" now is selected, then press the "Power key".
9. They phone should now shut off and restart.