It's pretty hard to determine without some more information. Could you give us:
–the WP version you're using
–the steps you're taking
–the error messages you're receiving
–recent changes you've made, plugins you've installed, WP updates, etc
If you upload plugins directly to the 'plugins' folder by ftp, are you able to activate them?
A good first troubleshooting step: deactivate all your plugins and try again. If it works with plugins deactivated, then reactivate them one at a time to determine where the conflict is occurring.