Desktop/laptop - it makes no diff, If the HSF is removed from the CPU the old thermo Paste must be removed and new paste applied. The old paste will be "dried out" and not fill in the the microscopic places where the HSF and the CPU contact.
(1) Reseat the CPU and then Reinstall the HSF on the CPU, Using NEW thermo compound.
(2) Check all power cables to the Motherboard (and Video card if it uses one) by disconnecting and reconnecting.
(3) Reseat all memory modules
(4) Disconnect all HDDs and CD/DVD drives, Both Power and data connectors, Disconnect Data connectors at mother board end.
(5) Try powering on and see if you can get into the BIOS Screen.
If you made it this far, Consider yourself Very lucky.
(6) If step 5 works, then power off and reconnect HDD. Power on and see if it will load operating system. Most likely the HDD is toast, but worth a try.
(7) Try reconnecting DVD drive. If HDD worked then reconnect DVD drive and check it out. If HDD (step 6) did NOT work disconnect HDD and try inserting your Windows installation disk - It will tell you no HDD found, But you will know if DVD drive works. Get a New HDD and reinstall Operating system