Any error message? Have you check if you have the updated version of Nokia Store? You can update it by going to store.nokia.com using your phone browser. The Nokia Store will prompt you to update the application if not yet updated.
Can you try the following troubleshooting steps:
1. Restore Factory settings:
End all active calls. Go to settings and restore factory sett. > settings only . Enter the security code. Default code is 12345 unless otherwise changed.
2. If still the same, try reinstalling the software. Connect the phone to Nokia suite, go to Software Updates section then click on the text reinstall.
Go to store.ovi.com from the phone browser. You should get an option to install the store app from there. If it still doesn't work then you need to contact the authorised Nokia Care center in you region. You can find one by going to www.nokia.com/support.