Are you talking about the separate 4 pin connection for CPU power? If so, it's a 12 volt connection. What are the old and new PSU's? What motherboard?
And now ...
You replaced the the old PSU with a new one, yes?
And the system didn't work.
Did you try replacing the old PSU?
A basic rule of troubleshooting:
If you are working on a formerly working system, and something goes wrong,
Stop. Put down the shovel and back away from the hole.
In other words, back up to the last working point. Did you reinstall the original PSU?
Does the system work with the original PSU?
Another good rule (while we are talking about rules):
When you are upgrading, upgrade and test one component at a time.