macpro 5.1 computer will not turn on

a very old problem, especially with Mac desktop computers, is related to the PRAM backup battery.

The very important parameters and the clock chip are normally maintained by an always-on (when AC power is available) small power supply. When you drop the AC power to your Mac, it forces the non-recharging long-life lithium 3 Volt coin cell to work to maintain the parameters. If that battery is too old, it may not be able to do that, allowing corruption to creep into the saved parameters.

This can cause the classic "nothing happens when I say power-on" symptoms. Users encounter this when they service or move their Mac to a new location, either of which removes AC power for a while.

When tested in hand (not in circuit) voltage of less than 3.00 Volts is considered 'spent'. Apple shipped the BR2032 battery, but users have found the readily-available CR2032 to be an adequate substitute.

After testing what memory?

Try reinstalling only the original memory modules in their original slots.

Also see the Memory Install Video @ https://eshop.macsales.com/installvideos/macpro_2010/?filter=memory