Sounds like what the error says, you have a XML file that is not well-formed (does not follow XML syntax).
Use a MPQ Editor and open your map, then drag ComponentList.SC2Components out of the map file and onto, say, your desktop. Now, open it with Notepad. Unless you know how to edit XML to make it well-formed, paste everything here and we'll help correct it for you.
For the sake of maybe helping future people with this issue, I just got the same issue and solved it. Using an MPQ editor (link at bottom of post) I tested it with an XML validator and everything was fine. So I looked to what I last changed with the UNIT tab in the data editor and found a comment I made. I named a unit after a historical person and copied his name from wikipieda into the editor. The guys name had an accented e. I opened up the map in an MPQ editor and replaced the accented e with a normal one and tried to open the map again normally. The map opened without a hitch and all my work was back.
I thought this was important enough to necro post this for any future issues anyone may encounter. Just watch out for any abnormal characters when editing any text fields.
You don't use auto backup? Sure its annoying as F when it decide's to save in middle of typing but it saved few hours of my work already.
set it to 2 files every 10 minutes and you won't lose anything next time ;)