I drive my 1990 xjs at least once a week and never had a starting problem till now.
Before I start mucking around thought I would ask for members help that have much more knowledge than I have.
Thank you for any advise.
Need more info … e.g. headlights still come on? Can you hear the fuel pump do its little startup “hum” when the key is first turned to the “on” position? Does the tachometer needle move more than just slightly while cranking? 
Fixed it, CPS cleaned prongs in plug.
k. The tach not moving (much) while cranking was/should have been your clue there. Yep, sometimes dirt/oil/road grime, etc. gets blown back or up onto the connector and if it gets onto the prongs or their holes you have a problemo. Glad you tracked it down. 