hi and welcome at MR! Sorry to read from your problems.
First thing. Do you really own a PRS-500? If so, PRSCustomizer 1.04 is, to my knowledge, for PRS-505
PRSCustomizer: A useful companion for your Sony PRS-505 Reader
Quote taken form the help-screen.
Though included UniversalFlasher also supports PRS500 with fw 1.0.03.07170
If, and only if the reader starts to run the AUTORUN-scripts on the MS/SD-Card you might be lucky to flash back the previous generated fsk.img. Its size should be 7340032 bytes for the PRS-500 and 8650752 for the 505.
Rename the backup to "new_opt.img" and copy it to the root of your card and run menu option 4.