I changed the boot mode from Legacy to UEFI and now my Windows 7 won't start, even after switching it back to Legacy. I switched to UEFI to see if I can boot from a SD card.
Instead it starts the Windows Boot Repair tool.
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On most desktop and laptop computers, "legacy mode" activates the Compatibility Support Module (CSM), but does not require that it be used -- that is, the CSM becomes a possible boot mode. (See this page of mine for more on the CSM and why it can be such a problem.) Switching to "UEFI mode," OTOH, usually deactivates the CSM, rendering a BIOS-mode boot impossible.
An EFI-based computer, whether it's booting with the CSM or without, generally relies on the EFI's built in boot manager to handle the boot order. This boot manager maintains a list of boot options and the order in which these boot options are tried. This is similar to the boot order of old BIOS-based computers, but more complex.
Combining these two issues, what's probably happened to you is that, when you de-activated the CSM, the boot order was changed to remove BIOS/CSM/legacy boot options; and when you re-activated the CSM, those BIOS boot options were either not added back or were added back in an order that's inappropriate for your installation. Either way, the solution is conceptually simple: You must adjust your boot order to place the appropriate BIOS boot option first in the list, or at least above whatever is launching (maybe the wrong disk or an EFI-mode boot option). In practice, you can usually change the boot order using the firmware setup utility -- the same tool you used to disable the CSM. Details vary greatly from one computer to another, though. It's also possible to adjust the boot order with tools like Linux's efibootmgr, Windows' bcdedit, the third-party (for Windows) EasyUEFI, or bcfg in an EFI shell; but to use any of these tools, you'll need to boot an OS (perhaps from an emergency disk) or at least an EFI shell.
There's a chance that something else is wrong. For instance, you may have somehow damaged your boot loader, partition table, or Windows filesystem. In such a case, you may need to run a Windows repair utility to fix the problem, but the details of what must be done depend on the source of the problem. More diagnostic information would be required to advise further. As I'm not a Windows expert, I can't suggest what specific tests might be required, much less how to fix the problem; but if adjusting the boot order doesn't help, perhaps somebody else will be able to provide better advice. Asking on a Windows forum might be helpful in this case, too. At a minimum, you should provide information on how many disks you have and how they're partitioned.
I will add that converting Windows to boot in EFI mode rather than in BIOS mode may be a solution. This is a radical option, though, so you should probably consider it only if nothing else works. For instructions, see this blog post.
