Exactly how to do it varies by model phone (google it) but do a recovery reboot and keep your finger's crossed yes backup is handy, but if the screen isn't working is sort of not possible. Reboot might fix it. It also might not. Other things to try include remove and replace the battery where possible wait at least 15 second's toput it back or disconnecting and reconnecting the glued in batteries... not with metal tools and very carefully mind you. Don't damage or short anything.