It happened because device communications between your blackberry and BIS server had stopped and has not restart.
It also means that your blackberry Id can't be read by the BIS server.
To settle this issue:
You must have a valid Blackberry Id.
(Every user must have a blackberry Id, created when using their blackberry phone for the first time. It is compulsory).
First, shut down your blackberry phone. Open the battery cover. Pull out the battery.
Then pull out your SIM card . Leave the SIM card out and insert back the battery to its place.
Restart/Switch On your device (usually device will automatically restart when the battery is reinserted).
Wait till the phone starts.
A screen appears asking you to key in your blackberry Id and password.
Key in your Blackberry Id and password.
Error message appears, stating you have 9 more try to key in the correct password bla bla bla. Ignore it.
Now, just shut down your device again.
Pull the battery out. Reinsert your SIM card.
Reinsert your battery.
Wait till the phone restart.
Find Blackberry Id (under option) or if you are using os7 - just type to search for blackberry Id.
Enter your correct Id (e-mail address and password).
The save it. (Click blackberry button, click save).
Now your blackberry should be working correctly and you can starts download from App World again.
Please ensure you have a data packet service from your telco provider.
If you don't have data packet service then you can only download through wifi.
Have a nice day.