Well if you are holding in the back button for about ten seconds and it is not turning off, then the button is broken and it will not turn back on which means you have bigger problems. In that case yes, you will need to pull the plug.
First, how long has it been at 90%? Over a few hours? If so then yes its stuck and I would advise you first use recovery in attempt to run install again. If it fails again, you will likely have to erase and reinstall the operating system. If you do not have a backup first this will be a problem. If you try the reinstall and it fails, and do not have a backup, let me know here. I can tell you how to use and external drive and either recovery or single user mode to do an rsync dump of data from your home folder to an external drive, at least salvaging data.
