My sansa won't turn on!!

Same thing happen to me. A Sansa is just similar to a cellphone.
Unscrew the backings and check if the battery is all the way in.
That fixed my problem, hope it fixed yours.

While plugged in, the unit receives its power from the computer, but once unplugged relies on the battery. The battery may not be making contact. To test, try pressing on the back to force the battery to make contact while pressing the power on button. If the unit powers up and you let go of the back and the unit goes off, the battery contact is the culprit. I simply took off the back cover and wedged wooden toothpick pieces along the bottom of the battery to force the battery upward onto the contacts. I also placed a toothpick over the battery so the cover would press down on it when the cover was replaced. Make sure cover screws are tight. This appears to have taken up the slack in the battery housing and solved the random power off problems - for now!

To get your screen to turn back on, try holding the power button for 15 seconds. the same thing happened to me and that helped me.

I can help you with the "bad track." That error message will come up once in a while. The files in your computr are not corrupted or anything: only the sansa's files are. Taking those bad tracks off won't help, you have to format it or something (I'm sorry, I forget excatly what it is!) but if you right-click on the sansa in My Computer theres OPEN, EXPLORE, and then FORMAT. This will wipe everything off of the mp3 player, but the problem you have with the bad tracks should be fixed. Just upload them again. I am really sorry I can't tell you how to fix your other problem, I'm having a similar problem with mine right now (I formatted it, then it stopped responding to my computer and won't show any image on the screen, just white letters).