To expand on whitesmith's suggestions, as a rule, Linux is far more stable than Windows because the GUI is just another app running on top of the base Linux OS, unlike Windows where the GUI is so heavily integrated into the kernel that a misbehaved app will bring down the entire Windows session.
Because the GUI and the entire desktop environment (DE) in Linux is basically just another app, there are many ways to show, suspend and even kill misbehaved apps, but the most powerful methods require the command line. Therefore, open a terminal and, to see running processes, do:To search for a specific process:
...and replace <process name> with the actual name of the process. Note the process ID (PID) beside the process name of interest.
ps -ef | grep -i <process name>
To terminate a frozen or misbehaving app, find the corresponding (PID) from the command just given and:...where <PID> is replaced with the actual numerical PID you noted earlier. A stronger version is ...but try "15" first and use "9' only if necessary. To learn some of the basic commands that are actually at the heart of Linux, this is a good resource.
If the whole DE is frozen, you can often reset things by hitting <Ctrl>+<Alt>+<Delete>. This does not reboot the system like it does in Windows. Instead, it forces a logout, which is clean, and puts you into the login screen. Starting with 10.04, I believe, the more powerful <Ctrl>+<Alt>+<Backspace> was disabled by default, but you can re-enable it by going into System Settings>Keyboard Layout>Options>"Key sequence to kill the X server" and activating the checkbox. This will allow you to kill your whole X session in future if your DE totally freezes up (you can't even bring up a terminal). You will lose any unsaved work, but the Linux system underneath will still be rock solid.
Alternatively, you can hit <Crtl>+<Alt>+<F1> to get to a pure bash shell command line console and kill/restart your DE from there with
Last but not least, if your computer is really gummed up, you can ssh into your frozen box from another computer altogether and initiate a complete shutdown or reboot. This technique is probably too involved for you at the moment, but it's nice to know that it's another option when you do have the time to explore it.
sudo service lightdm restart