I have an unknown message on my iMac telling me to turn it off, the computer has also frozen apart from that massage

i have an unknown message on my iMac telling me to turn it off, the computer has also frozen apart from that massage?

Sounds like a kernel panic.
What is a Kernel Panic?
http://support.apple.com/kb/TS3742
and  http://support.apple.com/kb/TS1892?viewlocale=en_US
http://developer.apple.com/mac/library/technotes/tn2002/tn2063.html
More details here:
http://www.thexlab.com/faqs/kernelpanics.html
Useful articles on how to avoid and eliminate kernel panics here:
http://reviews.cnet.com/8301-13727_7-10344626-263.html?tag=mfiredir
http://www.macworld.com/article/2027201/how-to-troubleshoot-a-kernel-panic.html# tk.nl_mwhelp

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