I'm interpreting this as "my phone got stolen, can they use the phone if they flash it". Yes, yes they can. Or if you found your phone, so can you. You should be able to factory reset using nokia tools and/or hard reset. If it is truly stolen and gone, you can the IEMI locked out by the carrier as well, which means they would have an ipod.
Edit; they would not have access to your data, the data would be wiped out.
A ordinary mortal can only gain access to your data when your device is unlocked. Most thieves don't care for the data and sell it as hardware parts or as a whole after a reset.