actually what may have happened is, you didn't deactivate it like you were supposed to, and then installed 7.55, there is however a way to fix this without havening to reinstall d2 completely, what you do is find your anticheat.dll, look and see if you have an anticheat.ce, if so, delete anticheat.dll and it should work perfectly, if it says something about any other files, look for the .dll versions of those and delete them. sure reinstalling d2 is easier but just making another suggestion^^, this happens because the uninstaller removes only the .ce files, and not the .dll,so when you install 7.55 the activator doesn't want to overwrite anticheat.dll so it screws up, and causes it not to work