This error (HTTP 500 Internal Server Error) means that the website you are visiting had a server problem which prevented the webpage from displaying.
This error can occur on all web sites, even on central admin. The root page default.aspx does not display at all, instead this error is shown. The virtual folders below the top site, like /_adm works fine.
If you experience this, make sure you don’t have changed any of the OOB files like /_layout/1033/init.js. Intentionally or not these files should not be customized.
Please note that there can be other reasons for this error message displaying.