I can't open my inbox?

My Nokia 6300 was made for vodafone and i changed its software to make it sim free, i've been using it for 10 months but i had a problem 2 days ago, i can't open my inbox or my memory card and when i switch it on it goes off from time to time and when someone rings me they found me out of service, i do care about the txt messages iin my inbox and i don't want to lose them. i'd like to get help how to open my inbox and if it's possible to transfer them to the memory card or the PC?