Sony ericsson v630i mobile phone problem

two things come to mind to check first:
take out the battery so you can see the SIM card. ensure that it is installed correctly and not loose in the restraining mechanism. In my older Motorola I had to use small pieces of paper to keep the dang thing tight so it wouldnt come loose. Once it comes loose, the phone usually errors and freezes or powers down.

second:=. make sure the battery is installed correctly, tightly, and all terminals clean, if they look a little corroded use some light sandpaper, or a gentle emery board to clean metal terminals

also, how old is the battery? it is possible it is either old or defective and not reporting its capacity properly to the phone.