Try holding the power and home buttons to get it to re-boot. Most likely that won't work but worth a try.
You'll have to put the phone in DFU mode and then restore in iTunes and re-jailbreak it
DFU mode
INSTRUCTIONS
Step 1. Turn off iPhone
Step 2. Hold down the power and home button.
Step 3. When you see the Apple logo, release the power button but continue holding the home button down.
Step 4. Open iTunes and wait for it to tell you that it detects iPhone in restore/recovery mode.
Alternative method.
Step 1. While the device is on, hold down the Sleep / Wake and Home buttons for exactly 10 seconds, the device will restart.
Step 2. Release the Sleep / Wake button, but keep holding down the Home button for another 10 seconds.
Step 3. Release the Home button.
Once it is in DFU mode, restore it in iTunes then jailbreak it.