Here is the link to the DELL Optiplex 790 built in 2011.
[link|https://www.dell.com/support/kbdoc/en-us/000126021/a-reference-guide-to-the-dell-optiplex-diagnostic-indicators#2009_to_2012|Click here]
With this series, the diagnostic LEDs include a combination of the Power button LED state and the diagnostic LED indicators. The Diagnostic LEDs remain 1, 2, 3, 4, and are placed on the front of the unit.
Let us know what is happening. Does only the power LED blink? What about the Diagnostic LED's.
The guide in the link above should tell you everything you need to know.
If you have any further questions on how to fix it please ask.