If your system has gone corrupt on your Nokia 5230 then check the user manual for more assistance which is on this website. You will also need a computer with a spare USB port to use. You will also need the Nokia PC Suite software to re-install the software on the phone.
Once you have the phone connected to your computer you can from the software on your computer re-install the operating system. Check first to see if you can backup the system of any of your files on the phone before you attempt a re-install.