Are they the correct region DVDs for your player?
Are they factory burned DVDs or homemade?
Are they really DVDs or are they video files such as .vcd?
Do DVDs that you already have and know work still play?
Is the TV/monitor input set correctly?
Are the cables still connected, and connected fully?
Not to sound condescending, but part of my job is fixing problems with AV equipment at a college and it seems like 90% of the time switches are not set correctly, cable not connected right, or somebody's trying to use the wrong type media in the wrong kind of device (Such as wondering why a CD with a PowerPoint presentation on it won't work . . . when it's stuck in a DVD player!).