Hi Michelle,
To start Python works on indentation. Your error indicates that you have a problem on Line 16, that is because the line above it is "try:" which means everything below it should be indented until you reach the error portion of the try statment, line 137 in your code. Your "if" and "else" statements, for loops and nested "try" statements all need to be indented.
I have attached a script I wrote that will go through subfolders looking for MXD's and seeing if any layers within the MXD have broken links and print that to a text file. It should help you with your script.
I hope this helps,
Cory