Vishnu, from your username I assume that you're from india. Which firmware did you download?

Was it:
RX-51_2009SE_20.2010.36-2.004_PR_COMBINED_004_ARM.bin 200043133 PR 1.3 version 20.2010.36-2 Latest Maemo 5 India release for Nokia N900

If not, this could be one reason why your sim isn't being recognised.

But before you go reflashing. Check that the contacts of your sim are making proper contact with the pins on your sim card holder. Similarly with the microsd car. The covers should depress and slide into a lock position.

If you have a card reader and another mobile, it's worth trying to test the sim and microsd card out in other devices to rule out that it's not a faulty sd card or sim.

Cheers,
Fred