sim card! no services! After screen replacment!

A problem occurred in my phone after the screen was replaced (no service)!  The person who does maintenance says, "The problem is from the body? And it was changed and the problem was not resolved .. Then he told me that the problem is from the outlet and did not carry the problem .. After that I took my phone and went) What is the solution please?  I followed all possible methods in the software, but I do not know the hardware!  And I think that there is a problem with the hardware!  I think I trusted someone who did not know what the hardware was!  Unfortunately ... I don't know what to do