In order to fix the issue, go to Applications > Utilities and open the Terminal. Run the sudo killall -KILL appleeventsd command and try again to use Finder. If it doesn't help, you will need to reinstall it. For this purpose, you can use the Pacifist software. Once you launch it, insert your OS installation disc, locate the Finder file and install it following the provided instructions. For further details, you can check the Apple forum.