It's amazing the ease with which you can completely screw over your basic Autocad interface, and require a phd in Autocad Science to bring it back to life.
I once renamed the <<Vanilla>> profile directly; it screwed up my autocad so bad the admin ended up completely re-installing Autocad, but I digress.
What's probably happened in your case, is that you've unloaded or removed your acad.cui file (or acadm.cui if you run it as Mechanical). You'll need to find that file somewhere on your hard drive.
On our XP systems running acad 2010, you would find this file here:
c:\Documents and Settings\yourname\application data\autodesk\autocad mechanical 2010\r18.0\support\acad.cuix
(Note 1: This is a hidden folder, so you will have to set Explorer to show hidden/system folders or you'll never find it. Note 2: With Autocad 2009 and later it's cuix files, not cui files)
To load this file, you type 'CUILOAD' at the command line, browse for this file and load it. Do the same for 'acetmain.cui' if you want to load Express Tools back up.
There are other files in this same folder that will be needed for the autocad interface to load properly. If these have been deleted, you'll have to get them from somewhere else.
We had this issue arise several times in this office, and what I did to fix the problem was to simply copy the entire contents of my Support directory to the computer that had the problem. That fixed the problem quite nicely.