Is there a cable to connect my windows laptop to my iMac 27

To do what?

Are you saying you want to use the Mac as an external display to your Windows laptop? If that is correct this is called Target Display Mode and ONLY works on 2009 & 2010 27" iMacs. Please read Target Display Mode 2009 & 2010 iMacs for all the information you need.

If your iMac is a mid 2011 machine then no you cannot connect it. Let us know the spec's on the machine then we can easily give you a more definitive answer.