Very slow and can't watch my videos

I'm trying to make a video for my dad, however whenever i go in to edit his video, it's super slow on the frames, i can only see one still image and all the audio at some points, sometimes it's just a black screen and no audio, the video just stops completely sometimes wether its just a black screen or on an acual frame from the video, what's going on! Is anybody able to help? Also, i put all the video's and images i was putting in the whole video into a folder on my desktop from just having everything everywhere on my desktop, and now all the video and images say that there is nothing connected and i have to find and connect the document. Is there anyway to counter act that?