upgraded from 3gs to 4s and when I syced the new phone 6 months of contact entries were left off? Backed up old phone on computer, but don't know where to find the info or how to get it back on new phone......??

Upgraded and switched to Verizon today.Salesperson to back up old phone and then sync to new one to save all contacts, and the contact entries from January to June didn't make the jump? Old phone is not available, but I did select the "backup to this computer" option when attempting to back it up. It took some time to complete, but I have no idea where the information is .....or how to get it on the new phone? Any help would be greatly appreciated.