SCT is way out of collimation how can I get it back into alignment?

My Meade 12 inch classic lx200 is so far out of collimation I cannot focus on anything.  my Zwo asi290mm mini  cannot reach focus. pictures from it are just blown out white.  no matter how I adjust the scope nothing can be seen (not even shadows).  How can I find a reference point between the focuser ,the corrector plate position and the secondary mirror position.  I am at an impasse.  Any ideas you may have are appreciated.  Tom Jackson