There is a black screen with slanted arrow icon

The arrow you may be seeing is the thunderbolt logo. If that is true, then your MacBook is most likely in target disk mode. You can find information on target disk mode here: Transfer files between two Mac computers using target disk mode - Apple Support

In order to use your MacBook, simply press and hold the power button until it turns off, and then restart it. Make sure to not press any keys while it is starting, just to make sure it boots normally. I hope this helps!