try downgrading (a process that may bring up various mysterious errors, so be prepared to do further research):
Download and open TinyUmbrella (ignoring any start-up messages), plug your device into your computer (let iTunes make a full sync and backup just in case), click "Save SHSH" in TinyUmbrella, and see if you have any SHSH blobs from firmware versions earlier than 4.3.4 — these blobs will allow you to restore to their corresponding version numbers. (You can try going to the "Advanced" tab and checking or unchecking "Request SHSH from Cydia" to try to find more blobs.)
Be sure to uncheck "Set hosts to Cydia on exit" and "Request SHSH from Cydia" before attempting the restore. If you are downgrading an iPhone 4 and are not concerned with preserving your baseband (for example, do not check this if you rely on ultrasn0w unlock), check "Update iPhone 4 baseband". Now ensure that iTunes is shut down, and your device is plugged into your computer.
Click "Start TSS Server". Now you can minimize TinyUmbrella, turn your device off, and then put it into DFU mode: hold the power (lock) button and home button for 10 seconds, then release the power button but keep holding the home button for another 15 seconds.
Open iTunes. Your device should be recognized immediately as being in recovery - if not, and if iTunes prompts you to check with the iPhone server, click "Check", and then your device should be recognized as being in recovery.
Then hold down the Shift key (Windows) or Option key (Mac), and click "Restore".
iTunes will ask you for the firmware file (also called "IPSW file") for the version that you want. You may be able to find this firmware file, named something like iPhone3,1_4.3.3_8J2_Restore.ipsw ("DeviceNameX,X_FirmwareVersion_XXX_Restore.ips w") on your computer, or you can just download a copy of the correct file using the links on The iPhone Wiki.
After the restore process finishes, you may get an error with the code "1015". This is expected - just go back to TinyUmbrella and click "Exit Recovery Mode". (If that does not work, you can put your device into DFU mode and try the "Fix Recovery" button in TinyUmbrella, and if you are still stuck in recovery, download the stand-alone "fixrecovery" tool from the TinyUmbrella website. You will get two files: one is 421 (for 4.2.1 firmware), and one is fixrecovery43 which you should use if you are downgrading to 4.3.3. You may get an error mentioning that you need "zlib1.dll" - download it here and place it in the fixrecovery folder.)
Your device should now reboot and be normal. You can now jailbreak your device again! Or you may have run into various other errors during this process - try searching JailbreakQA, because somebody else has probably run into the same problem before you.
