Hi! I was getting the error "Boot device not found, Please install an Operating System". But when I restarted my computer,...

Hi! I was getting the same error but when I restarted my computer, it started working fine. But I need to know what could have happened and what measures should I take now to prevent it from happening again? Also, my laptop is getting stuck for, like, milliseconds so I am scared that it might happen again. The first time it happened, my screen got stuck then I got the message that there are some errors. When it restarted, I got that black screen. Please respond soon and let me know what I should do now!