Q: New update froze and now computer will not start up

Reinstall El Capitan Without Erasing Drive

 

       Please be sure you back up.

 

1. Restart the computer. Immediately after the chime hold down the Command and R keys until the Utility Menu appears.
2. Select Disk Utility and click on the Continue button.
3. Select the indented (usually, Macintosh HD) volume entry from the side list.
4. Click on the First Aid icon in the toolbar. Wait until the Done button activates, then click on it.
5. Quit Disk Utility and return to the Utility Menu.
6. Select Reinstall OS X and click on the Continue button.

 

Also, see this User Tip: Basic Steps for Upgrading OS X.

It's the first key to the left of the Space bar. It should have "command" on the keycap. Otherwise, you aren't using a Mac keyboard.

Same location. If you are going to use a non-Apple keyboard, then you need to know what keys match on each.

If  PC keyboard then use for the Apple Command the Control or Windows/Start key

https://support.microsoft.com/en-us/kb/970299

In addition to the 'microsoft help link' already posted, there is an Apple support

article here that also has some helpful items for both Windows & OS X in Mac.

• Use your Apple Keyboard in Windows with Boot Camp - Apple Support

{Microsoft provides keyboard mapping article how to use a Windows keyboard w/ OS X}

Some of this has dual purpose, in that BootCamp allows one to run Windows

in a Mac, so to know which keys perform various tasks on Mac keyboard is

as useful in the reverse circumstance. Apple function key equivalent as well

as Windows keys are shown in the above.

These details are covered between both articles - I've kept these bookmarked. Yet

I've almost always made sure to have at least one as-new Apple keyboard on hand.

Perhaps by now, hours later, you may have found the workaround.

Good luck in any event...!

If you are not using a wired KB try using a wired KB.

Otherwise try another wired KB