Mac Pro wont recognize Apple monitor

hi,
run dual monitors mac pro (first intel version)
have samsung , old apple monitor.
worked fine until disconnected fix tangled cord issue.

put , apple monitor recognized screen black. (yes, double-checked make sure it's on.)

know mouse travels black screen because opened finder window did not show on samsung screen , after 'fishing' able drag black screen samsung screen.

old apple monitor uses vga port.
appreciated.

 

just swirched inputs

switched monitor inputs in of mac pro.
samsung still works t apple monitor detected, still black. button on front of apple monitor in bottom right lit up, when press on sun looking emblem on lower left there no response.

 

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