Do you have an iPhone or iPad that you cannot access? Is it locked by a passcode that you have forgotten? If so, you will need to use Recovery Mode to regain access to this device. Apple’s support article describes how to use Recovery Mode. Here are some important caveats:
If you have previously backed up the iPad or iPhone to your Mac or to iCloud, then you’ll be able to recover your data from the most recent backup once you use Recovery Mode to regain access to the device.
If you never backed up the device then all of your data will be lost.
If the device had its Find My iPhone (or iPad) service enabled then you’ll also need to enter the AppleID password associated with the iPhone or iPad as part of the process to regain access to the device. Everybody should enable the Find My iPhone feature since it can act as a theft deterrent. If this feature it enabled then thieves will not be able to erase iPhones or iPads to sell them. If thieves find that every iPhone or iPad they steal is protected by Find My iPhone then, hopefully, they will be deterred from stealing them in the first place.
I have used Recovery Mode over half a dozen times in the past few years. It has worked every time except for the most recent time. I’m not sure why it was not successful in this instance. I was working on an iPhone 4 or 4S and I was unable to force restart it. I recommended that my client take it to the Genius Bar to see if they could assist further.