You mentioned in one of your emails that you have T100MD+ PLC which is EOL 8 years ago.
Anyway, it sounds like either someone has put a password in the PLC or the EEPROM is corrupted or damaged. You can try to use TRiLOGI software to transfer a program into the PLC and it will ask you if you want to delete the password and select yet. After you have transferred the program you can turn power off to ON again and see if the new program works. If the new program is not retained then the EEPROM is damaged and you may need to purchase a replacement.