This could mean a few things. Here are the different ways to try to fix it:
1. Charge your battery for at least an hour, preferably overnight. If that doesn't get rid of the red slash, continue down this list...
2. Try a fully-charged battery from another phone. Either borrow a battery from someone or go to a local store to try one. If the red slash goes away, your battery is defective. If you have had your Blackberry for less than a year, your service provider should replace it at no cost. The number for warranty service is: 1-800-801-1101.
3. Try your battery in another phone. Again, if your battery does not work in the other phone, your battery is defective. If your battery does work in the other phone, you have a software issue.
You need to download the latest version for your device. You can get that either from your service provider's web site or from Blackberry (www.blackberry.com).
Save the folder to your desktop, double click it to install it your computer - be patient - it can take a while.
After you have downloaded the software to your desktop, connect your blackberry. Go into the Desktop Manager on your computer and use the program called Application Loader to install the new software on your Blackberry. Make sure that, when you see the "Advanced" box, you choose that option. Make sure you check both boxes to remove all current installed software.
4. Finally, if reloading the software does not work, likely your Blackberry is defective. You should call your service provider and ask that it be replaced (Note: call them even if you are out of the warranty period; if you are a good customer, they will often replace your phone anyway).
Hope that helps.