Jude,
Yes the source is still on the computer. I put the computer to sleep and went out of the office for awhile. When I returned some of the files that were previously unrendered became rendered. I checked my User Preferences and Auto Rendering is on. But if the clip was unrenderable manually, why would it respond to the automatic function (assuming that's what happened). And I have other clips in the sequence that I'm still unable to render.This problem is becoming more and more mysterious. Thanks for your input.