I have just the title of the site all things disaper

is a very unspecific description of your problem. So unspecified that it's impossible to give you any advice.

But you can:

1 - Disable all activated 3rd party plugins.

2 - Check your server's error log.

3 - Learn docs.

thanks for. I have this error:InvalidParameterException

As RvR already said, we need more details. So, please post the complete error message (error:InvalidParameterException would only be part of it) so we know in which file the error occurs. Also try what RvR suggested regarding the plugins: start with disabling all 3rd party plugins (the plugins you added in addition to the plugins that come with Elgg) to see if the error still occurs. If not, you already know that at least one of the 3rd party plugins causes the error. Then you would enable the plugins again - but only one at a time - to find out from which plugin the error is caused. Right now I have no idea what exactly you have done for the error to start occuring and therefore it's impossible to give you an advice how to fix it.

thanks for the reply; when i delete groups and some users the error InvalidParameterException start occuring.

This is not the complete error message. Check the error log on the server. The error message in the log should be more detailed.

When deleting users or groups all the content added by the user or added to this group respectively should also get deleted automatically. But if you had some 3rd party plugin on your site and there was added some content handled by this plugin either by the user or the group you delete and at the time of deletion this plugin is not enabled, it could happen that this content is not deleted. So, there might remain some content that belonged to this user (or group) and this content now causes a problem even if it wouldn't get displayed anymore.

Without the complete error message it's difficult to say if the problem is caused by some content belonging to a disabled plugin. You would know best if you had some plugin installed that now is no longer available and might cause the problem. In this case it might help to add this plugin again at least temporarily to delete any content that's still there from this plugin. Or it might help to use the dbvalidator plugin (https://github.com/Elgg/dbvalidator) to clean up the database from faulty entries (if you try this plugin, first do a validate run only to see if there is anything the plugin can fix at all and if you would make a fix run do so only after you have made a backup of the database).