Re-installing MODX can actually do wonders for your broken site. There are many things that affect the manager's usability, while your front end works just fine. Thankfully, a broken manager rarely results in a broken website.
Your best choice is to install MODX manually. Often your server will offer an installation script, which can be great or really screw you up. Re-installing MODX is a manual process, and the steps should be followed specifically.
The update information is found at Upgrade or re-install (reinstall) your MODX site)
Re-install (reinstalling MODX) is your friend. Re-running setup often does the trick, although a fresh set of files never hurts.
New copies of the MODX-versionYouWant.zip can save your day. I don't know how many times I've finally resorted to downloading a new copy of the file I used previously to install or update MODX and found that my download was obviously corrupted. The new version fixed it all. Hours lost to not bothering to simply fetch a fresh copy. Just go get it for every re-installation you try. I will try to do as I preach.
Remember to clear (delete) the core/cache files any time you run an update or re-install. At times, a browser cache is necessary but not always.
Please, if you have any comments about the documentation referenced above let me know what could be changed.