The Samsung GT-C3520 is a classic feature phone (part of the "Champ" series) running on a proprietary Samsung operating system, not Android. Over time, these devices can suffer from software issues like boot loops, hanging on the logo, forgotten passwords, or IMEI null problems. The solution is often to "flash" the device using a specific flash file (firmware) and a compatible flashing tool.
Since direct download links cannot be provided here (copyright/security reasons), search using these exact terms on Google or firmware archives: samsung gtc3520 flash file and tool
Alternatives to flashing