Gray screen when restarting

That screen appears when a Mac is unable to find an input device (mouse or trackpad). That's a concern because your Mac has a built-in trackpad, so you should never encounter it.

Exhaust all the options in If your Mac doesn't start up all the way - Apple Support.

Since the images you describe are not shown in that document, please follow the instructions linked in "Learn what to do if your Mac starts up to a blank screen, such as an empty gray, black, or blue screen."

There are a number of possible remedies so be sure to research all of them. If you reach the end without resolving anything, they conclude with:

"If you still need help, please contact Apple Support." 

Apple may want you to schedule an appointment to have it fixed.

In the mean time you can try using a wired USB Keyboard & mouse to use the laptop.