How to Unbrick your iPhone
hey i am sorry if i keep bothering u with this but u are the only one to be able to help this much. i am so lost i am not very smart when it comes to stuff like this . so if there is anyway u would be able to set the file up and email it to me i would very much be in your debt. 
i will even donate if need be i am just desperate
Hmm, well if I remember correctly from back when I was modding my iPod's firmware there may be some legal issues with uploading firmware. It would be too big to email any way. But even if I uploaded it to mediafire, etc I feel like that was always warned against. Sorry.
i have an iphone 4s and i got the new absinthe jailbreak. it worked fine until i turned it off. now it wont turn back on. i still have the apple logo though
Did you install any jailbreak apps that aren't compatible with iOS 5? If so and you know which one you can use iPhone Browser to navigate to the folder where that app lives. Rename the dylib file from app name.dylid to app name.backup or anything really. Then reboot. Unistall tha app through Cydia.
Now keep in mind I don't know this info to be fact. It is the best option I found researching your problem. I haven't had any problems with my 4S yet and the 4S jailbreak is still pretty new.
I did the right jailbreak. Which jailbreak did you use?
how do i know which ipsw file to use for the 3gs? used most recent and earliest and it says not compatible. i'm using windows xp. never had something jailbroke before. Got this from nephew. someone else jailbroke it. I was told nothing and tried to sync and update. so stuck in recovery mode. have downloaded lots of unusable crap to netbook trying to fix it. We paid $100 for this thing and just want to use it for tmobile with internet. i"m not dumb just lost.
From what I understand you attempted to update your iPhone 3G(S) to the newest firmware iOS 5.1 using iTunes. Unless you saved your SHSH blobs prior to updating you will be stuck at 5.1 as I am unaware of a way to downgrade from 5.1 without SHS blobs. So that means you will need to fix the problem to get you up and running on 5.1 then go beg AT&T to unlock it to use with T-Mo. Good news is AT&T is apparently unlocking off contract iPhones these days.
Open iTunes. If you can turn off the iPhone do so. If not try this anyway. Plug in the iPhone using the USB cable. Hold power and Home until the silver apple appears then count to 10. Release the power button but keep holding the home button. After a few more seconds iTunes should recognize the iPhone and ask you to restore it. Do that. Do not interupt the process. If that fails go to youtube and search for a video that shows you how to put the phone into DFU mode or Recovery mode. DFU is preferred.
thanks for your reply. Someone else jailbroke it so hopefully they have the files. At one point I got error 1015 saying i was downgrading but when i used the most recent it said not compatible. Oh well. Maybe the orginial person can fix it. Thanx again for taking the time to reply. No one has on the other websites. 
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Download the ipsw from the link below that matches your phone.
Download IPSW
Make a copy of the ipsw and put one in a folder on your desktop.
Follow the first link & do each step to get the iPhone into DFU mode.
How To Enter DFU Mode Without Home Or Power Button Tutorial
Download Sn0wbreeze and make a custom ipsw from the ipsw you copied into the folder on your desktop.
Download Sn0wbreeze
Use iTunes to restore your iPhone with the custom ipsw using shift + restore.
DFU is an important tool in jailbreaking. If you can get into DFU you can restore which should solve the problem of being stuck at the boot logo.