SGS doesn't go to download, recovery mode nor does it connect via USB after installing a battery MOD I installed some app called "Galaxy s battery icon mod 2" on my rooted Samsung Galaxy S (with a lagfix OCLF), it required rebooting my handset.
Then, it didn't boot up, my handset entered an infinite loop, (after the Samsung galaxy S GT-I9000 then the galaxy S logo appears) it vibrates as it does when some app hangs or freezes, some mouse cursor on the upper left side, and I can't even get it to enter recovery or download mode using the three buttons, but all what it does is restarting the phone and the loop begins again.
Tried to connect it via USB, didn't connect or anything.
Does anyone know what to do before resorting to the jig factory mode method.  Ah, and USB debugging mode isn't activated, and some times I get a recovery screen, guess it got something to do with the app, and it says enter key or something but I can't enter anything or selecvt anything on the screen, it got recover and cancel.
When an SGS won't go into recovery mode, there's a couple things you can try.  One thing is to hold all the buttons to get into recovery or download mode (VolDown + Power + Home, or VolUp + Power + Home, or VolDown + VolUp + Power depending on your model) until the device has reset 2 or 3 times, and then release Power so that the device stops resetting.  Hopefully then it will go into the appropriate mode.  If not, attempt again but this time do it while the device is plugged into USB.

Another thing to try is to remove the battery, then insert it while holding the above buttons.  You may want to try multiple resets combined with this as well, as well as having it plugged into USB.

Once you get it into recovery mode, you can try restoring your backup and/or doing a factory reset and clearing the Dalvik cache.  If you get it into download mode, I recommend using Odin to flash a stock ROM as per this question's answer: Can only access 2.2GB of internal storage from Windows. Just use an i9000 ROM, not the Captivate ROMs linked in that questions.  You can find one at http://forum.xda-developers.com/showthread.php?t=846913, for example the newest Europe/Middle East/Asia one is I9000XWJS8.