Please read the following and if your problem cannot be resolved then feel free to post in the support forums. Make sure that you include important troubleshooting information.
- Check error logs in server.
- Switch to a WordPress Default Theme like the Twenty Fourteen or Twenty Thirteen themes, etc. to rule out possible issue/s caused by your theme.
- Deactivate all plugins in your Plugins Activated panel except BuddyPress to see if that resolves the problem. Remember to also deactivate plugins in the wp-content/mu-plugins folder as well as any custom /code in your wp-content/plugins/bp-custom.php in your server if any. If the problem is resolved at any stage, reactivate plugins one at a time until you identify the plugin/s which caused the issuue.
- If you cannot access your wp-admin/dashboard, you can reset the plugins folder via FTP or phpMyAdmin.
- Clear server and/or browser caches.
- Download a fresh copy of the latest WordPress and BuddyPress versions and re-upload to server.
For BP theme-related troubleshooting, go to https://buddypress.org/support/topic/basic-troubleshooting-tips/