sounds like an easy solution to this one. it might help if you could tell us which version of gnome and also which distro of linux you are using.
As a guess i would say gnome is no longer loading a file manager.
The easiest way to fix this is as follows. open the gnome control centre, and find the bit where it mentions startup programs. you need to load the gnome file manager (i think the command is gmc but im not sure as i use kde
now restart gnome.
If it doesnt work, try logging in as another user. if it works ok then it means the options for the particular user are bad
delete them but loggin in as the problem user and opening a terminal.
type the following:
rm -rf ~/.gnome*
Then restart gnome by logging out and back in. you may need to fix up a few things for which the options will have been reset but otherwise it will be working.
If this doesnt work then i would say the issue is more complex and serious and more fundamental.