Look under Help menu then About Mach3...... when you click a window will open telling you the version and whether its in Demo mode or not. If its licensed it should have your name there (or the name of the licence)
If it says Demo then do a search of your drive for a file called Mach1Lic.dat, when you find it place it in the main Mach3 Directory and restart Mach.
If you cant find it email licence@machsupport.com and Scott will likely issue you with another licence, as long as you can prove you previously had one.
Hood