I wish you had not used Secure Erase as it is not needed. A normal fast erase takes just a few seconds. The former can take hours or days depending on how many files there are to remove.
After the drive has been erased you will regain control. Until then do not try to do anything with the computer. When the computer can be used, again, then you need to start by installing Snow Leopard from the disc that came with your computer when it was new. Should have been two DVDs with gray labels. They may be marked as Software Restore discs. You will need the one labeled as Disc One. Follow these instructions:
Snow Leopard Erase and Install
Be sure you back up your files.
- Insert Snow Leopard DVD into the optical drive and restart the computer.
- Immediately upon hearing the chime hold down the C key.
- Release the key when the Apple logo appears, and wait for the loading to finish.
- After the installer, loads select your language and click on the Continue button. When the menu bar appears select Disk Utility from the Utilities menu. After Disk Utility loads select the out-dented disk entry from the side list (mfgr.'s ID and drive size.) Click on the Partition tab in the Disk Utility’s right window. Set the number of partitions to one (1) from the drop down menu. Click on the Options button and select GUID, click on OK, set the format type to MacOS Extended, Journaled. Finally, click on the Apply button.
- After formatting has finished quit Disk Utility. Continue with the OS X installation and follow the directions.
- When the installation has finished, the computer will restart into the Setup Assistant. After you complete the Setup Assistant you will be running a fresh installation of Snow Leopard. Open Software Update and install the recommended updates. You can stop here because you need not do anything more with Snow Leopard.
- Download and install Mac OS X 10.6.8 Update Combo v1.1.
At this point, you should be running Snow Leopard 10.6.8. Now, you can download and install El Capitan. You can re-download the El Capitan installer you downloaded when you originally installed it. Open the App Store, click on the Purchases tab in the App Store's menubar. Locate your original download entry and click on the Download button on the right of the entry. The installer will automatically run when it finishes downloading. This may take a while as it is a large file - over 6GBs. When the installer starts it will open an Install button on your screen. Click it to begin the installation.