Dell OptiPlex 780 with Windows 7 shut down

It sounds like you are experiencing an issue with your Dell OptiPlex 780 computer where it is shutting down unexpectedly. There are a few potential causes for this issue, and several potential solutions that you can try to resolve it.
Here are a few steps you can try:

1. Check for updates: Make sure that your computer is up to date with the latest Windows updates. Sometimes, these updates can fix issues with the operating system that may be causing your computer to shut down unexpectedly.
2. Check for hardware issues: Make sure that all of your computer's hardware is properly installed and functioning correctly. If you have recently installed any new hardware, it's possible that it could be causing the issue.
3. Check for malware: Run a malware scan to make sure that your computer is not infected with any malicious software. If you find any malware, remove it as it could be causing the shutdown issue.
4. Check for overheating: Make sure that your computer's internal temperature is not too high. If it is, it could be causing the shutdown issue. You can try cleaning the inside of your computer and making sure that the air vents are not blocked to help keep the temperature down.
5. Check the power supply: Make sure that your computer is receiving a stable power supply. If it's not, it could be causing the shutdown issue.