Sounds like something has bitten you. So we have to start at the beginning.
Check your files, are the moodledata files in place? Go and look, if they are, then there is something in the Moodle not working. If not, then check your environment, check your server logs. If you are on a shared host, then check your data/space limits. Have you gone over and the files you are uploading are not actually making it to your site? Can you do something else to increase your space limits if this is what the issue is? If not, then read on.
The database is the next place to check. Open your database, if using MySQL or PostGres, try downloading and installing phpMyAdmin, or phpPGAdmin, it is an excellent tool. Go to the table mdl_course_categories and check to see if the categories yu are supposed to have are all there.
Next, go to mdl_courses and see if the courses you should have are there.If they are, then go to the next step, if not then replace your code as outlined below and reinstall your Moodle - carefully, without wrecking your moodledata folder OR the config.php file.
Use your FTP program and download the config.php file. Open it and have a look at the opening lines. It should read :
<?php
unset($CFG);
.....
There should not be anything between the first two lines. (For good measure, you may also want to go to the end of the file and delete the ?> from there. This is not essential and it can clear up a couple of other problems.) If there was anything else, then save the file, upload it and replace the one that was there. Try it again, and see if you can access things.
From your 1.9.16 test Moodle, and you should have one and if you do not, create one on your local machine, look at the file structure that controls the upload and downloading of files. (This should be a working Moodle with a few courses and a number of different users.) Compare that to the one on your production server. If they are different, then you have an issue, so delete the one on your server and upload the one from your working installation. Try to see if it is all working as it should.
We could spend ages tracking down a small issue, but it may be better to just replace the "admin/*.php" folder completely.
As an alternative, it may just be better to go to this discussion and follow the instructions there. This is probably the fastest way of dealing with a lot of problems, but if the issue is in the database, then no matter what you do, nothing will change.
