PC Workshop has seen all sorts of Mac problems over the last couple of years, but one we tend to see most of is listed today which in certain cases is the most worrying as it wont allow users to retrieve data easily!
A flashing folder with a question mark at startup time means that the machine cannot find a valid boot volume.
This doesn’t necessarily mean that the hardware is broken; it may just be that the contents of the internal drive has become corrupt or erased.
One way to recover is to use the recovery partition. That only works if whatever trashed your boot volume didn’t also trash the recovery partition.
Another way to recover is by reinstalling Mac OS X from the Internet, but as you found out, that only works if you can make a network connection.
If you have an external USB hard drive with a bootable copy of Mac OS X on it (e.g., a backup that you created with a cloning program), you might try booting from it.
If that fails call PC Workshop we will recover your data and restore your Mac back to perfect working order!!