Template and Theme Disappeared

Hi..Ive very noob to this. Just started yesterday. I had gone to the appearance setting and there was a tab for ‘Install Themes’ I installed a theme, activated it and did a trial post. Everything looked fine. I called it a day, planning on coming back to it to continue adding header/etc but when I logged in a ‘default theme’ had been set, the post i had created was gone, and there was no longer an option in the Appearance section to add a theme. (additionally, the number of themes available went from 9 pages to 4). I have no idea whats going on or what i did wrong. Any suggestions? Thank you for your help!