Today I have unlocked my Nexus S's bootloader. After unlocking, all data was wiped from the phone, however, I was not aware that the sdcard would be wiped along with the system data. Assuming my files were saved, I rebooted my phone to realize that, in fact, the sdcard had been formatted. I am here to ask if there is any chance of recovering the lost partition even though android system wrote files to the sdcard after format.
Thanks in advance.

Depends on how it was formatted. If the sectors were overwritten then your out of luck.
Download & install winhex on a pc then attach the sdcard.
Start winhex & then open tools>open disk & select your sdcard
Then look at the sectors to see if they have been zero'd or if a pattern has been written.
You could also try rstuido data recovery software

This article explains what to do in your case: Photo Recovery. R-Undelete is based on the same data recovery engine as R-Studio, but easy in use.

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R-Studio Data Recovery Software

Thank you for the responses. After looking at the device in WinHex, I discovered that 0s have been written over the data. I tried R-Undelete anyway, but as I expected, it did not retrieve any data. Thanks anyway.