Have you changed media recently (i.e bought a new batch of DVD+r discs when you ran out of them)?
If one media works the next one might not, and on top of that even if you buy the same brand and same type of package the next time it may not work. Well that happens atleast if you are using cheap media.
Also you can consider yourself pretty lucky to be able to use DVD+r in your xbox. My xbox can't handle them...
If the problem occur with previously working movies too it might be your DVD-reader thats giving up on you. Or the media you are using goes bad after about 6 months I know for sure my "Konco" branded DVD's did... Same with some of my xeed discs...
