Hi Cornish,
You really have to check your error logs for the specific error. Assumedly, you changed some configuration while you were using LSWS and Apache doesn't like that configuration now that you've changed back. (LSWS does not change Apache configurations. If there was a change, it must have been done manually.) A 500 error, though, covers an extremely broad swath of server problems.
Perhaps we should have suggested this earlier, but maybe you would be best suited to using a managed hosting provider? If you wish to use LSWS, there are a number of hosting providers listed on our site. Most of them offer managed hosting. We do not offer free support with our software, but a managed provider would give you support, of course.
Sincerely,
Michael