If you are having issues with your BlackBerry such as your call logs or SMS/Text messages suddenly disappearing, the information in this thread will help you clear up the immediate issues facing your device.
Other symptoms of low-memory on your BlackBerry include losing pictures that reside in your device memory, losing emails within the time period it is set to keep messages on the device, and often simply shutting down and rebooting.
To do first:
On your BlackBerry, go to Options > Status. Check the "File Free" entry. The number is expressed in Bytes, so a number there of "10123456" would indicate you have 10.12 MegaBytes of memory free to use on your BB.
While there is not hard and fast rule of how much "File Free", free memory, is required, the symptoms above are most noticed when the free memory is below 10MB. The lower the number, the more likely you will experience loss of call logs, SMS and email messages. It is recommended to strive to attain a file free higher than 15MB.
To do second:
Removing the battery on your BlackBerry and replacing it, will often clear up between 2-10MB of free memory. This is commonly referred to as a "reset" of the BlackBerry, and is recommended by RIM for circumstances such as these. Doing so will not delete any settings, applications, data, or personal informaton on the device. Rather, it simply clears the cached memory of the BlackBerry and releases memory to be free that has been in use by other applications and not yet released.
With your BlackBerry remaining powered on, remove the back cover, remove the battery and hold about 30 seconds. Replace the battery and allow the device to reboot. Check the file free status again, at Options > Status. You will notice a marked difference.
Following the clean-up tips in these FAQs posted below will help you short-term:
How do I free up memory on my device? - BlackBerryFAQ.com
Additional information on memory leaks on the 8100 - BlackBerryFAQ.com
Why is my BlackBerry losing its call logs or message logs? - BlackBerryFAQ.com
For long-term support on memory issues, please read this thread on Optimizing Your Memory.
Optimize your BB! How To Setup for max free memory!
What you can't do to increase device memory:
a. You cannot direct your SMS/MMS/email/messages to be stored on your Media Card.
b. You cannot load applications onto your Media Card. They must be operating from the device memory.