Try booting it again (hold the power button down until it restarts). If it still starts in download mode (which is what you're describing), you'll have to reflash the firmware (Nexus firmware is at https://developers.google.com/android/images - agree to the terms and you'll get the whole page - scroll way down to the nexus you want). Instructions are at the top of the page. (Make sure your computer has USB 2,0 or USB 3.1 ports - some Google phones don't work with 3.0 ports. [I just wasted the entire day trying to update my Pixel 2 because the back ports on my desktop are supposed to be 2.0 - they're not.])