If the device is still under warranty, I would take it in for warranty repair. If not, I would try the following:
1. Plug the phone into the official BlackBerry wall charger for 2-3 hours to make sure that they battery isn't just dead.
2. If you can try a different battery, consider doing so, again, just to make sure it isn't just the battery.
3. If you still can't get the device to reboot, do you at least see a red LED shortly after you reinsert the battery into the phone? If so, a software reload may help. In that case, follow the instructions in the following link:
Note: If the loader.exe file cannot see your BlackBerry at the end of Step 2, remove the battery and unplug the phone. Then plug the phone back into your computer (without the battery). As soon as you see your PIN or PIN:UNKNOWN or something similar pop up, click Next. If you miss it, no worries, just unplug the phone and plug it back in - it will likely take ~30 seconds to pop up.
Once you've made it to step 3, you can reinsert the battery and proceed with the OS reload process.
I hope this info helps!
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