I have a SanDisk Sansa

Make sure the lock is not engaged first:

These are the most common methods for recovering Sandisk's Sansa Mp3 Players. I have been almost universally successful in recovering Sansa players using one or a combination of these methods. These methods do not work if you are having synchronization or music format issues - they are for recovering a player that will not power up, freezes, flashes, is not recognized by the computer, and a host of other issues.

Reset the Sansa Player first. On most models, restart is done by holding the power button in for 15 seconds or longer. Check your specific manual to be sure. The manuals are available from the Sansa website if they have been misplaced.
Download the Sansa Recovery Tool from the Sansa website.
Format - Erases Everything: Connect your Sansa player to the computer and open Windows Media Player. Click on the Sync tab. Pull down the drop down menu underneath the Sync tab by moving your cursor over the Sync tab and left clicking on the little black arrow underneath the Sync tab. Highlight "Sansa 1GB" (wording may vary slightly), and then select "Format." - this should erase everything.
Firmware Upgrade- Erases Everything: Get the latest firmware upgrade from Sansa - it updates the firmware and erases every song in the player all at once. Note: this method doesn't work if already have the latest firmware. Go to the Sandisk website and click "Firmware Updater" to find out.
If these fail, then:
Try this: disconnect the player from the computer, turn it off, hold in the volume + button, and reconnect the player to the computer while holding the volume + button in. Keep holding the volume + button for an additional 5-50 seconds. Windows should eventually go into recovery mode and revive the player- it will pop up a window that says Found New Hardware Wizard.
If you get the problem where MTP Device keeps popping up continuously, uninstall the “MTP Device” by going to Start à Control Panel àSystem à Hardware à Device Manager (you computer may vary slightly, depending on Windows Version).
You may have to restart your computer after each of these methods to get them to work properly. Disconnect your Sansa player BEFORE shutting down your computer.
-Tha Mp3 Doctor