How can we help?
When a theme is missing it usually because of a conflict somewhere. There are 2 ways to resolve a missing theme:
On your Dashboard, disable all your plugins, if your theme is displaying properly, enable them one by one and check your website after each is enabled. As soon as your theme disappears, you will find the conflicting plugin.
You can then look for a plugin with similar functionality, or contact the plugin developer to investigate it for you.
On your Dashboard, change your Theme to one of the standard ones, if your website displays, then it is possible that either there is a problem with the theme you were using, or you have done a customization that has caused the glitch.
In the case of the latter you will have to undo the changes and try a different approach, or double check your coding.