So now you've jailbroken your iphone and nothing happens when you turn it on. ok, first take your iphone that you just broke, email me, I will send you my address and send you money for shipping to my house.
Seriously, I will do that if you can't figure this out below

Now, seriously Bricked iPhones are simple to unbrick

1. First turn on the iPhone, wait 10 seconds (don't pay attention to what the iPhone screen says on it, it doesn't matter)

2. Open itunes and plug your cable into the iPhone

3. Now hold down the power button and Home button at same time until the screen shuts, off wait 5 seconds after the screen shuts off to let go of the power button, only Keep home button pressed for about 30 seconds

4. a popup in itunes will say "iPhone is in restore mode and must be restored."

5. Hold down the shift key while clicking restore, goto your .ipsw file you saved on your PC (you could have one from your iPhone before you bricked it or a downloaded file), ignore any error messages at this time on the iPhone

6. phone should say connect to itunes

7. run jail break

and there ya go !

Note: if You don't want to have it jailbroken be sure to save a copy of the .ispw file to your PC now

Also These instructions are for PC, NOT MAC
Since I have not tried this on a mac I wont guarantee it works on a mac, but I guess you can get mac itunes to restore by trying this:

Hold down Option/ALT-key while clicking restore goto your .ipswfile you saved on your PC (you could have one from your iPhone before you bricked it or a downloaded file), ignore any error messages at this time on the iPhone

the rest is supposed to be the same !