A white screen is rather common and not a sign of a bad repair. Sometimes, when installing new hardware, the phone doesn't immediately recognize the new parts you have put into the phone. By performing a hard reset, you essentially re-boot the phone so all electronics and parts are operating on an even footing. Simply hold down your power button and home button at the same time for anywhere between 45 seconds and a minute and a half.
*When performing a hard reset, make sure that the device IS NOT plugged into your computer as this will wipe your data. If you do a hard reset with the phone simply in your hands and not plugged into anything, your data will be completely fine, it will just re-boot the phone.
If a hard reset does not work the first time, please try it again once or twice. If you are still experiencing issues, please email firstname.lastname@example.org or call us so one of our support agents may better assist you.