It can happen, that your EFS partition got corrupted. If it's true and you don't have a backup of it, then you cannot restore it and probably your IMEI is also lost. The next command (for a Nexus) presumably makes a backup image of efs, which can then be checked for consistency:
dd if=/dev/block/platform/omap/omap_hsmmc.0/by-name/efs of=/sdcard/efs.img
The next command makes a backup of your baseband/radio image:
dd if=/dev/block/platform/omap/omap_hsmmc.0/by-name/radio bs=4096 of=/mnt/sdcard/radio.img
If you exchange the parameters of "if" and "of", you can flash/restore a radio image.
you can check the next folder for device files as well:
Bootloader, baseband/radio firmware images can be found on this link:
If you have a i9023 device, you'll need to check out section "Factory Images "soju" for Nexus S (worldwide version, i9020t and i9023)". Please unpack the aforementioned img-s from the zip.
You should check the /dev/block/platform/omap/omap_hsmmc.0/by-name/ folder for block devices containing firmware. Though most partitions containing firmware images can be hidden by default, but they still can be accessed and modified (with restoring purposes as well) through a serial console. A serial console with UART can be activated through your USB port by connecting appropriate resistors to some pins.
If your further attempts to flash a proper baseband image are not successful, then one of the last hopes is enabling serial console.