Step 1

Disable all plugins and try enabling plugins, one by one, while you refresh the problematic page after each plugin enabled, until you find the source of problem.
If no plugin was the cause, go to step 2.

Step 2

Edit your current theme’s functions.php file and comment all occurrences of

add_action();
add_filter();

function calls, like this:

# add_action();
# add_filter();

Then try to uncomment them one by one, while you refresh the problematic page after each line uncommented, until you find the source of problem.