Can't install El Capitan and now I'm stuck

Do yo have a bootable clone of another OSX version?

You can try booting to Recovery

OS X: About OS X Recovery - Apple Support

And try reinstall the OS via Recovery. Nothng shuld be lost from the HD if yo do not formate the drive.

Please try again after taking each of the following steps that you haven't already taken.

Step 1

Reset your computer’s PRAM.

Step 2

If possible, set the system clock as instructed here. If you can only start up in Recovery mode, see below.

Select Get Help Online. Safari will launch. While in Recovery, you'll have no access to your bookmarks, but you won't need them. Load this web page.

Triple-click anywhere in the line below to select it:

date 0801000014

Copy the selected text to the Clipboard by pressing the key combination command-C.

Quit Safari. You'll be returned to the OS X Utilities screen.

Select

Utilities ▹ Terminal

from the menu bar. A Terminal window will open. Paste into the window by pressing the key combination command-V.

Wait for a new line ending in a dollar sign ($) to appear below what you entered. If it doesn't appear, press return.

Quit Terminal to return to the main screen. Try the installation again.

Step 3

If your model has user-replaceable memory, and you've upgraded the memory modules, reinstall the original memory and see whether there's any improvement. Be careful not to touch the gold contacts. Clean them with a mild solvent such as rubbing alcohol. Aftermarket memory must exactly match the technical specifications of the machine.

Step 4

Back up all data to at least two different storage devices, if you haven't already done so. One backup is not enough to be safe. The backups can be made with Time Machine or with Disk Utility. Preferably both.

Erase and install OS X. This operation will destroy all data on the startup volume, so you had be better be sure of the backups.