SD/TF Card Error?

When I turn on my NDS Lite with the R4, the screen goes black, then 'SD/TF Card Error' pops up on the top screen.
What's going on?! And what can I do to fix it?
I just played my R4 about eight hours ago, and it was working fine...

Try backing up, check with chkdsk (Properties of MicroSD) if you want, and then reformat your MicroSD.