Answered question
I have no access to the mobile network. A message appears on the screen "insert your sim card". How can I solve this problem?
Try -remove your card and reinsert it
-insert your card to another phone
-insert another phone card in yours.
If the SIM has a problem you will need to contact Fizz to ask for a replacement SIM. Otherwise you'll have to replace or repair your phone.
Other answers
Did you try to remove the sim card and reinsert, it seems to be a problem with the contact or the sim