Won't recognize music files

The laptop and the computer may have had two different file systems (NTFS vs. FAT) - plugging the mp3 player into the other computer caused a mild reformatting. I would start simple - go into the Setting Menu of your Sansa and change the USB mode away from where it is now - if it is in MTP Mode, change it to MSC and vice versa (or Auto is even better). See if the missing files now show up. There is a full user manual for your product available at Sandisk's website if you want to get a visual on what I am talking about.

-Tha Mp3 Doctor

Here is a link to the manual:
http://www.sandisk.com/Assets/File/Downloads/user-guides/c200-ug-eng.pdf

If you are certain that all of the files are in mp3 format, then your best bet would be to change the USB Mode of the player - if it is in MSC Mode, chnge it to MTP Mode and vice versa - instructions for doing this are in the manual.

-Tha Mp3 Doctor

Use Windows Media Player to get the ripped songs from CD onto the player.

It is probably because the files are not .mp3 or .wma format files. This player is not compatible with itunes's .m4a files. 

make sure the file is MP3 or WMA, and not copy right protected.

is it on or off when you plug it in? try both.

you could also try a different computer, only two of the 3 comps in my house even recognize my player.

can not get music videos on my mp3 player

We have two laptops and one desktop in my house and two 2GB Sansa c250s. My laptop recognizes my sansa c250 differently than our family's desktop (i.e. on the desktop i can't view certain folders/ music files that i transferred from my laptop to my player). The second laptop doesn't recognize mine at all, but it DOES recognize my sister's sansa c250.
my suggestion? try a different computer! and then KEEP using that computer for adding/ deleting files.

Open device manager under the Control Panel to view hardware and devices. There should be a tab under portable devices that recognizes the Sansa c250. If their is a yellow exclamation point, then most likely the device has stopped working. Right click on it, and uninstall the device. Unplug your Sansa, wait a couple seconds, then plug it back in. Let it do its thing. If this does not solve your problem, then you should deffinately call the manufacturer.

THE ONLY HELP THAT I CAN GIVE YOU IS TO CALL THE COMPANY HERE IS THERE NUMBER Contact Customer Support at 1(866)SAN.DISK