Inbox deleted

Question

1-m gettin a letter 'E' overmy ntwk on the top left corner of my display screen..
2 mnths back i inserted my sim card in my new cell..3 days back all my inbox msgs got deleted except for the msgs which were saved atht tym on sim card..i.e 2 mnths back..how come only those msgs dint get deleted..and how my inbox msgs gt deleted in its first place.