Task manager keyboard shortcut in Linux?

I am going to assume by "my machine go into crashed state" you mean that whatever task is taking up the display you are looking at has stopped responding. (In general, when something crashes on Linux, only that thing crashes and everything else keeps running. It's very rare that the entire machine comes to a halt.)

When all else fails, I like to switch back to a standard terminal interface (text mode as opposed to GUI) by hitting CTRL+Alt+F1. This brings up a login prompt. I then login, and enter the command top to see what is running. The process at the top of the list is the one using the most CPU and usually the problem, so I kill it by pressing k, and entering the process ID (the numbers on the left). I then go back to the GUI by pressing CTRL+Alt+F7 (or sometimes CTRL+Alt+F8, one of those two will work, but it might change). If things are now working, I continue on, if not, I'll try again or may just force a reboot.

Here are a few useful shortcuts you can try:

• <Ctrl><Esc> displays table of processes
• <Ctrl><Alt><Esc> converts the pointer to a skull-and-crossbones and will kill the process of the window you click on
• <Ctrl><Alt><Backspace> kills the X-server
• <Ctrl><Alt><Del> shutdown the system and reboot

It's slightly related, but if you're dealing with a crashed system, you might want to invoke the Magic Sysrq key. This way you can kill all processes, sync your disks, print out the active tasks, initiate a crash dump, and much more.