Try to restore my iphone 3gs but an unkown error occured (-1), what does it mean? how can I restore my phone?
Force your phone in to DFU mode and then restore it (if iTunes allow you to) and setup as new.
Serach in google "force iphone in to DFU mode"
If you manage to restore it successfully, avoid making illegal modification(s) afterwards.
Retrieving data ...