I have laptop installed with 12.04 and one day when I powered it up, it showed the option of booting from ubuntu and advanced etc. On selecting boot from ubuntu, it showed the blank screen. i tried restarting re-seating the RAM/HDD but no joy. then re-installed 12.04 now i want to upgrade to 13.04 however I came to know that i have to get 12.10 first. Do I need to download 12.10 and only then can I upgrade?
Yes, you'll need to upgrade to 12.10, then to 13.04. You don't need to specifically download the ISO for 12.10 though, you can upgrade over the network. What you will do is open a terminal (Ctrl+Alt+T) and type
Then, you'll follow almost the same steps when you are on 12.10. Ubuntu Desktops 12.10 to 13.04.
Alternatively, since you've just re-installed 12.04, why don't you just do a fresh 13.04 install? It will be less of a hassle, and you'll have a more stable distribution rather than upgrading from one to the other.