“500 internal server error” after changing theme

I just want to have a new theme for my WP blog & was trying few of themes one by one. While changing the theme to “Snow `08 AM” i got “500 internal server error”. Now my whole blog is giving the same error including my admin. Am not able to login to admin theme page too.

Is there any way to solve this issue without losing anything in my DB? Am using the latest WP version.