Plugin update screws up site

Hi,
I think I’ve run into this problem before but it’s happened again and I’m not sure why it keeps happening or what to do to fix it. My admin panel said that I had to update the Contact Form 7 plugin so I did. It said there was an error (“couldn’t copy file”) and when I now look at the list of plugins the Contact Form 7 one isn’t there. My live site now has the slideshow missing, the contact accordion menu doesn’t open, thumbnails don’t show up in galleries, etc. Please let me know what I can do in this situation. Thanks!