It won't press the button even though the power is on
Since you don't indicate what you mean by "won't work", let me provide a few possibilities:
1) You close the door and you can hear the DVD spin, but no reaction on the screen other than some type of "loading" message. This indicates that the optics are attempting to read the DVD but are having difficulties. The operation may ultimately be successful and the DVD may start, but the movie will be unwatchable because of freezes etc. In this case, try cleaning the lens using a cotton swab and alcohol. A dirty lens can cause this.
2) You close the lid and do not hear the DVD spin. In this case, the optics can not even determine that a disc is in the machine. Again attempt to clean the lens. If that fails to at least cause the disc to spin, the optics have failed and need to be replaced.
3) Try a music CD. If that plays OK, but a DVD does not, then the optics are defective and need to be replaced. DVDs require a more powerful laser and will typically fail sooner than CD levels.
Hope this helps,