Welcome to the forums.
It sounds like you are very low on memory.
What is your file free Options > Status?
Pull the battery and check again. Post your results back here.
Also, follow these links to prevent this issue from happening.
Why is my BlackBerry losing its call logs or message logs? - BlackBerryFAQ
How do I free up memory on my device? - BlackBerryFAQ
Follow Kevinwska's steps about the memory card, that will help significantly. To prevent this issue in the future, follow the steps in the link skyman84 posted.