FOr me too having the same problem. am getting a feeling like i want to throw it into dust bin. how to solve this. it is been 1yr only. i bought my mobile last august. but for first one month it worked fine. even i change sim when i go home town. it will works. all of sudden. one fine day it got locked to one airtel sim which is am using mostly. now a days when i go home when i change my sim. it wont work. i mean my phone shows symbol that it is going to be switched ON. but it wil not ON . i kept it for a whole day. but it didnt got ON. i don know wat to do. >> any solution please??? And another thing is i cannot able to configure gmail box. in my mail box settings. it wil ask for manual configuration. for the first time when i bought my mobile facebook widget , mail box. everythng worked fine. but suddenly i got that sim changing problem right. from that time onwards these things too stopped working. this is my new phone 
the same problem happen to me..but i ve just find a way to solve it..
i'd used simcard MAXIS HOTLINK (MALAYSIA) with my J20i for a year then
when i try to use another simcard, my phone cannot be start..what i do to recover this prolem is, i ve just turn on the pin code for the new sim card using another phone then i placed it back into J20i.For sure, when i turn on the phone it will ask for the pin code..then i ve just simply enter the pin code and its working...PROBLEM SOLVED!!! 
:smileylaugh:..
After getting my phone unlocked and it still failing to start with the new SIM I'd almost given up until I found this post suggesting to use the SIM lock on another phone first...worked a treat. Thanks
