Don't panic..lol
Maybe you could list your computer specifications: Mobo, CPU, ram, hd, vid card, etc. This will help us to help you.
When I have had similar probs, I disconnect everything that I can to get to a base setup and try to boot. If it boots, add components (turn off, add a component, restart) until I get hung up again.. and that is what is causing the trouble. Don't know if you can, but worth a try.
Also, since the last thing you did was add paste, inspect your CPU well. You may have squished out a bit of paste on a side and got it on the cpu pins. If you did, it's conducting pin to pin and causing problems. Clean with qtips and alcohol and BE VERY CAREFUL NOT TO BEND ANY PINS. Clean the mobo cpu holder too if it got on it. Make sure your pin alignment is good and reinstall.
You can remove cpu with fan on it, just be careful.. or you can re-paste again.