If you have been using the DVD player regularly and it was showing no signs of giving trouble then it may be a number of things.
1) Was the DVD you put in compatible? Was it a copied DVD? Some old DVD players may only play DVD-R, others DVD+R.
2) If it was a copy DVD, was it finalised in the DVD Recorder used to create it? If not, then it won't play/be recognised in any standalone DVD player.
3) Is the DVD dirty? If its badly smudged or scratched then it might not be seen by the player.
4) If it was getting used regularly then perhaps the laser is dead on it, but normally you would notice it giving some bother before it actually packed in.
DVD players can be had from the likes of Tesco for €20 now, so hardly a cotly purchase if you need to replace it.