If you receive ‘500 Internal Server Error’ then it may be because of any one of following reasons
Your web server may be configured incorrectly (ie., configured with incorrect directives). You have to contact your hosting support providers to resolve this issue.
Your files may be corrupted while uploading into your web server. Those files may be transferred successfully, but it is unusable by server. If it is theme files, upload it again and import demo content then. If you still receive this error, deactivate the plugins one by one. So that, you can find the cause of errors thereby.