Changing your BIOS drive type from IDE to AHCI and vice versa will result in being unable to boot windows. This action requires a re-installation of Windows. AHCI is the recommended interface type.
Without knowing which version of Windows you had running, the interface between the external drive and your computer (you said SATA, did you mean e-SATA or was it on a USB adapter?) and if an OS had also been installed to the external drive, I can only speculate at your problem. It appears likely that your BIOS had been set to boot from external media, which tried to load an OS which wasn't configured to run on the hardware that this new (your computer) device had. Earlier versions of Windows like to add drives to the Master Boot Record and reacts poorly when one is removed, often refusing to boot without them present in the case of external and multiple drive computers.
Since there isn't a question I'm going to assume you'd like help in getting the computer running again. If there is no data that you need recovered, I recommend resetting the BIOS to default, then selecting AHCI as the SATA interface, then installing a fresh copy of windows (with all unnecessary media such as USB storage and external drives removed). Once complete, disable booting from removable media in the BIOS.
If you believe you need to fix the corrupt copy of windows, set the BIOS back to the same way it was before you modified it and run the windows repair disk, or select "repair my computer" during Windows installation.
If you are unable to repair as the MBR has become too corrupt, you can use data recovery tools such as Recuva on another machine to extract the data from the desired hard disk, then perform a fresh install of windows as described above.