Auto deleting songs we wanted there arg...

If you are using Windows Media Player to synchronise the MP3 player, you need to change the Sync settings, from auto to manual.

If the Sync option is set to Automatic, then Windows Media Player will synchronise your PC and your player, so they have the same tracks. Unfortunately, if a track is deleted from the PC or the MP3 player, then Media Player will delete it from the other.
However, Media Player may have only deleted the tracks from its library, and not from the PC. If you go to My Documents ----> My Music, you can see if the files are still there.

The Manual sync setting allows you to drag and drop files from the Media Player library on to your player as you wish.

Assuming you are using Media Player, connect the MP3 player to the PC.

Open Windows Media Player.

Right-click the Sync tab and select Tools -----> Options.

Click on the Devices tab and select your player from the list then click properties.

In the window that pops up, click on settings, then click the button next to manual to change from auto.

Click on Finish, then close the other windows by clicking on Apply or Ok, until you are back to the main Media Player screen.

You should now be able to transfer your tracks manually.

If this doesn't work, give more details, including make and model of player, and the software you are using for transfer, and I'll try to help further.