You need to put your phone in DFU-mode and than restore.
Hold home and power button together, after about 7 seconds the phone will turn off, keep holding home and power for another 3 seconds, now release the power button while holding the home button for another 10 seconds.
The phone should be off with a completely darkscreen, itunes should give a message that your phone is in recovery-mode and should be restored.
You can now restore your phone by hitting restore or you can download the IPSW firmware and select it manual by holding ALT(mac) or SHIFT(win) and click on restore.
IPSW firmware can be downloaded from here:
sorry my english is really bad, is it right, that i cannot re-restore 5.1.1 on my iphone 4s to 5.1.1, but i can restore to an earlier version?!
maybe 5.1?!? with the new redsn0w 0.9.15b3
it seems i have a ota update (5.1.1) in my iphone 4s:-/ unfortunately is cydia out of order, neither i can install anything nor a i delete cydia files :-(