Hello, and welcome to JustAnswer.com!My name is Jesse, and I'm here to help.
Does any disc show, or is it a specific disc that's not working?
It will not even load and it is a disc that i have used many times in the past... it is not even taken in the drive mechanism.
Try reseting the PRAM, by holding the P+R+Option+Command keys together while restarting. Hold them for 3 full restarts.
Did so with no luck...
How old is your Mac Pro?
So this is not the tray model, that you place the disc into, but the model that grabs the disc from you?
Try restarting while holding down the Option key. Once you reach the disc selection screen, try inserting the disc.
Any other ideas?
At this point, that confirms the drive has gone bad and will have to be replaced. It's a fairly cheap repair on that model, and you might be able to do it yourself, if you're comfortable opening the machine.
I can provide replacement instructions if you'd like.