using a macbook pro. days ago there was a recent update on sierra. when i restarted it I got the grey screen with stop sign in the center. tried restarting smc and vram but it ends up with the grey screen. what to do?

Your internal drive or physical drive is the out-dented entry. Your startup volume is the indented entry. As stated it will usually be Macintosh HD unless you renamed it or it is non-existent. For the above, you would need to select the indented entry even if it is named Untitled.

The above is an example of a typical entry in Disk Utility. The indented entry here is, Macintosh HD. Can be different on your computer.

Here you have a typical entry for an APFS formatted disk that has two volume partitions. The indented entries of relevance are macOS HS T... and High Sierra.

Install El Capitan or Later from Scratch

Backup if possible before continuing.

1. Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:
2. Select Disk Utility from the Utility Menu and click on Continue button.
3. When Disk Utility loads select the volume (indented entry, usually Macintosh HD) from the Device list.
4. Click on the Erase icon in Disk Utility's main window. A panel will drop down.
5. Set the Format type to Mac OS Extended (Journaled.)
6. Click on the Apply button, then wait for the Done button to activate and click on it.
7. Quit Disk Utility and return to the Utility Menu.
8. Select Install OS X and click on the Continue button.

Thank you, but how can I back-up?

Glad I could be of service. Happy Holidays!!