Ok, enough about me. 
As ditzy said, your question is really vague.
There are a few reasons why your computer can't play DVD.
1) You do not have a DVD drive. A CD-ROM drive can't play a DVD.
You need at the least, a CD-RW/DVD-ROM combo drive. Anything lesser
than this you can't play your DVD.
2) You have a DVD drive, and have tested other DVDs, and they can't
play as well. Some DVD drives requires you to install some drivers,
which comes together when you buy the DVD drive. If you have lose
the CD, please go online to find them.
3) You have a DVD drive, and have tested other DVDs, and they can
play. Only this particular DVD can't play. This means that the DVD
could be spoilt.
4) Media players can't play it. Please download the relevant
codecs. Some DVDs come with protection, which means only a
particular media player can play it. The media player will usually
auto-run and install on your computer. If you accidentally
uninstall it, please re-install it. Even with you download codecs,
the DVD will just not recognize it because the correct player isn't
used.