Hi MacAttackAZ
Is this something that happened only after updating to 5.6.3?
If so, from previous cases that match your description of the problem, we know that it may be caused by plugins you have installed becoming out of date and losing compatibility with the new version.
If you have Motion set to open to the project browser, then Motion can't open until all the plugins you have installed for Motion and for FCP are loaded. An unresponsive plugin will prevent this process from being completed.
Try removing plugins - especially anything from Pixel Film Studios, then see if Motion opens.
Other advice has been that you should check that you have at least 15% storage available (BenB).
Or, is this just happening when you are opening a specific motion template? If so, then that template is likely to have a pairing of behaviors, usually link behaviors, that while running fine in the template, will create a 'link loop' that Motion cannot process again when saved - and prevents the template from being opened again.