Yes you have a linux, what distro: Ubuntu, Debian, Fedora ? Well anyway at what point does the black screen come and system freeze? If you can tell us more what was the last event obervable before it occurred.
Assuming it is you X server getting lazy, try login via terminal (CLI, Dos prompt, whatever you call it) when screen hangs, Do this:
Press Ctl+Alt+F2 (or any combination with Ctl+Alt+F3,F4,F5,F6) until a login prompt shows on screen that looks like ----
"Welcome to Linux 2.6.xxxxx"
<Here enter your username and password, then hit Enter>
You should have logged in say like: (you have dollar $ beside the prompt)
Then issue command:
Look for a backup file of xorg.conf like: "~xorg.conf" or "xorg.conf.bak" this you shall need to restore your Xwindow configuration: Do this--
sudo cp ~xorg.conf xorg.conf
chmod 750 xorg.conf
Press Ctl+Alt+Del to reboot. You should see X window running then.
If your system does not have an xorg.conf file you may create one and configure. Use manual pages.
--$ man xorgconfig
A manual page will brief you what to do.
Hope this helps.