Used Disk Cleanup, now my Win7 won't boot

I have a Windows7 machine (I know it's old, it's running some legacy stuff). It was running out of disk space, so I used its Disk Cleanup tool to clean up some old files. As a result, it stopped letting me log on, and when I restarted it, it now won't boot, saying MissingOsLoader. 1) how do I get back to a bootable system, and 2) is Disk Cleanup not safe to use?!

Disk Cleanup is usually very safe to use, so it's possible that some new hardware error has occurred (especially with an old disk).

To recover, you will need a Windows 7 boot media of the same service pack level as is currently installed, or a System Repair Disc if you have ever created one for your computer.

You need to boot from this media and do Startup Repair.

For detailed instructions with screenshots see How to Run a Startup Repair in Windows 7.