sharp dvd/cd player help!

I have a Sharp "DVD Micro System" model # XL-DV60. It is a DVD/CD player. It often has trouble playing DVD's I think because it gets clogged up/ confused when I've been switching out CD's and DVD's. But recently, any time I've tried to play a DVD or CD-any one- it will play for about 3 minutes then it turns off and on the display screen it says "PROTECT". It is REALLY frustrating! I've looked through my entire manual and I can't figure out why it would do this. Does anyone have any insight into this problem or have any idea how I can fix it so I can watch movies and listen to music? Thanks so so much!