I don't know if this issue was discussed previously or not. Here is the case, my bro owns a S2 i9100G (non-rooted) and performs OTA whenever there is latest firmware released by samsung officially. Now, he is running on android 2.3.6 DZKJ2 kernal. Out of sudden, the pc couldnt detect the phone when it's connected via USB. When we tried to enable USB debugging hoping that it helps to connect the USB mass storage mode rather than the MTP mode. However, it still fails to do so. The phone is charging when it's connected to the USB port and this certified that the port of his phone is not having any problem. We searched through the web and encountered many ppl will face this kind of situation in 2.3.6.

We managed to find the solution in XDA forum and hoping that this little trick could help someone with stock rom to solve their problem if they encounter similar problem.

The steps are:
1. Press *#7284# and go into PhoneUtil
2. There are 2 options under UART and USB, the MODEM and PDA
3. Now, please check the USB if the setting is set to PDA. If you having difficulties to connect USB like me, please select MODEM and then select back to USB again. (Tap the Modem then tap back to USB)
4. Then, you quit the phone util by pressing back button.
5. Try to connect the phone to pc again and by now MTP mode / USB mass storage (if usb debugging is enabled) is come back to live again.

It will be great if TS can share it in the FAQ post in the first page.

Well, then it's good. The first page indicated that reinstalling the USB driver will helps. I'm not a regular in s2 thread and have no idea if it's been shared before. Anyway, thanks