Why is my Blackberry Curve playing up?

The other day my blackberry restarted randomly, then after it did it said error restart... so I did then it did the same continuously for about 4 times. Then it eventually came on but at the top it said 'sim card error' then changed to 'intialisation failed'. It eventually worked, but then I turned it off at night turned it back on and now it's saying the same sim card error and that... I haven't dropped it recently so I don't know what's making it play up? Thanks.