No picture

Lately I cannot get a picture when I turn on the DVD operation--the TV screen, as always, is on INPUT (Extension).....the unit show PLAY, but the picture does not appear--and the TV screen stays blue! The audio is on, but no video....
This is something rather recent. However, if I unplug the electric-power source for the DVD-player, then sometimes the picture does appear, though not always.
I have checked the manual but find no good answer there. Please let me know what to do....many thanks.