You see lots of Galaxy S phones on ebay with the no signal problem but there is just no information on how to fix it, well i wanted my phone fixed so it was a case of do it yourself again so here is the first and at the moment only fix infomation on the internet on how to fix the no signal problem. I can't say if this will work for you but included is a 30 second check to see if your galaxy S has this problem.
I think most people just think its imei barred but the Galaxy S (or mine) still shows a signal with any sim even imei barred.
Remove the battery cover
Notice the small external antenna port, this i found is like a jack plug and if a antenna is plugged into it then the internal antenna is disconnected, i found this had gone open circuit and connection to the internal antenna is broken producing no signal.
To test if this is your fault get a small piece of wire and put it into the socket as shown, if you now get a signal then your socket is faulty and needs to be removed.
Disassemble phone and remove socket, then solder a link across where the socket was to connect your internal antenna