Second time this has happened. I go to open an email, and the little mail icon changes to open but it doesn't open the mail. Last time this happened I had to remove the battery, is this normal? I googled it and found nothing.
You are low on memory, if that number gets below 12,xxx,xxx memory cleaner will automatically kick in and the first things to go are emails, call logs, SMS's. It does this to free up memory. What OS are you running? Go to Options >About. It will be 4.3.0 or 4.5.0