How to get back should be easy. The system has not written any damaging data to your hard drive. It simply can't understand its data stream this way.
You need to enter the BIOS Setup screen system and make the change there. On most machines, you do this by holding down the "Del" key while it boots up. After a few lines of info in the POST sequence, it will suddenly put you into the opening screen of BIOS Setup. When you do this, watch the screen closely (it changes quickly), often near the bottom. If the "Del" key was the wrong one, you'll see a prompt about which way to enter Setup.
The Setup system is a bunch of menus where you can see and change parameters. There will be prompts on the screen, usually to the right and bottom, about what keys to use to move around, how to change some things, and what your options are. Find the menu page (probably by moving across the top level of tabs) where the SATA hard drives are configured. On that page you will see the ports and at least one, maybe all, will be Enabled. Close to that will be the port mode, usually setable individually for each port. Choices may include IDE (or PATA) Emulation, SATA, AHCI, or RAID. Set your port to IDE emulation as it was originally. Now look at the bottom for the key to Save and Exit - often F10. Confirm and the system will save your settings and reboot that way. Should work smoothly now.
Just FYI, in many places in the BIOS Setup screens there are key settings that can disable your machine and make it hard to recover. There are also lots of place, like the one you just changed, that make a difference but can be changed with no problem and then changed back again if necessary. I generally try not to change anything I don't understand. To help, read your mobo manual. Among other things, it usually will tell you how to recover from a really bad situation by resetting the BIOS to fatcory defaults that work, just so you can get back in to fix things. But you rarely need that - I've never had to. It's just there is case....