It can take a long time to get a GPS fix for the first time on a phone, either when new or after repair, as the memory will have been wiped. Leave the phone somewhere with a good view of as much sky as possible, with the GPS active for half an hour, and it will download the satellite data again. Then next time it should get a fix more quickly.
If your phone has an application for assisted GPS that downloads the satellite data from the internet, try running this.
Sometimes, there can also just be bad times for getting a fix, if the satellite geometry is bad.