I downloaded a couple of videos, and when I try to play them there is no video, only audio.
Does anyone know why this may be happening ?

Which player are you using? Maybe you need codecs for your player.

Check the thread below for links to codec packs (only have one codec pack installed on your computer at a time or you can run into problems) & also VLC player... it's a free download & has codecs installed to play most file types. CODECs list for Movies/Audio/Alternate players / Video help forums/ video-mp3 fixers

windows media player,
& what is a codec ?

CODEC and Codec - Wikipedia, the free encyclopedia
Codecs allows your wmp to read music & video formats.
We usually recommend VLC player because it has codecs built into the player itself & plays almost all video formats you will find on the internet.

oh thankyou !
but my main concern was whether or not I'll be able to put them on my ipod.

Not easy to know until you have downloaded them. More about iPod requirements here How to transfer to my iPod (& iTunes) - for video. There are some converters (some are free) around to convert to iPod ready size & format. iPod only accepts mpeg 4 & H.264 format videos, but also must be within a certain size, etc.

Oh, I see.
Thank you for the help, I can watch it now !
: )

Many thanks for this tip - I had exactly the same problem and this is now fixed - I'm away to watch True Blood now !! finally !