My site was obliterated while testing out themes

i had just downloaded the most recent version of wordpress and began testing out themes on the site. 30 minutes later a message appeared, informing me that the session had expired and that i could re-enter my log in and return to the site. i did re-enter the login and was thrown off the site. i returned to find it gone. a simple text page with a portion of my features remained, no theme, no control panel, no dashboard. what happened? what do i do?