Hi

Probably because:

1.  You are importing a video/audio clip(s) which require a codec which you have not activated before.  OR

2.  You have upgraded your computer reinstalled MEP and not reactivated all the codecs you use.  OR

3.  You have used registry cleaning software which is not very well behaved when it comes to installed software and it has wiped out the registration data for the codec.

Until you give us more details it is difficult to pin down the cause.

John