update to 10.13.2 statred & screen shows a circle with a line thru it. What can I do to get computer to reboot ???

Prohibitory symbol

When you see a circle with a slash symbol instead of the Apple logo, it means your Mac couldn't find a valid System Folder to start up from.

If you're using your Mac at a school or business, it might be trying to start from the wrong version of macOS. Contact your IT department for more help.

If this is your personal Mac, try reinstalling macOS using macOS recovery.

Just ran into this issue when upgrading to 10.13.2.

Something about this upgrade seems to mess up the Startup Manager.

*** SO.... Before you go reinstalling your OS X, etc., try this...

Use Startup Manager

When you use Startup Manager to select a startup disk, your Mac starts up from that disk once, then returns to using the disk selected in Startup Disk preferences.

1. Hold down the Option key immediately after turning on or restarting your Mac.
2. Release the Option key when you see the Startup Manager window.
   If your Mac is protected by a firmware password, you can release the key when you're asked to enter the password.
3. Select your startup disk, then click the arrow under its icon, or press Return.
   If you press and hold the Control key during this step, your selection is saved in Startup Disk preferences, so it persists until you change it.

See https://support.apple.com/en-us/HT202796

If your Mac boots, then go into System Prefs > Startup Disk to double check that you indeed have the correct Startup Disk chosen. Unlock, make sure you choose your correct Startup Disk, then lock.

This solution prevented the same issue from reoccurring on future restarts. All good now.

Hi, when my partner's Mac updated she started getting the symbol. After trying many things, I started it again holding Cmd + r and got the disk utility screen. Somewhere, I can't remember where on the dropdowns, is an option to start from disk. The icon showed that it was the latest version and when I clicked it started installing the update. What I think happened is that my partner had downloaded the update then closed the Mac to carry it somewhere else before the update had been installed or while it was installing. When she woke it up again the Mac couldn't find the OS to start up from because it hadn't been installed yet. Anyway, worked for me.