now a lot of investors are investing in bitcoin, therefore bitcoin has increased very rapidly, if bitcoin decreased what should be done?, whether the investors will lose? 
Do you mean if bitcoin drops? If so, well all I can suggest is to just hold it in. Hold it until the next bull run. I think we have witnessed bitcoin dramatically rise during its highest peak last December 2017. This proves that bitcoin, above all cryptocoins, has the highest potential and it is never impossible that the history will repeat itself.