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SIM went bad? Not likely, but possible if it's old.
Phone's SIM slot went bad?
SIM somehow shifted? (this is likely if you put a smaller SIM than the device requires in)
Or, the weird thing my tablet did -- it just wants you to toggle airplane mode?
There's a large number of possibilities it could be. The easiest troubleshooting is to take a SIM from another device and swap to see what happens and help you determine the exact cause. If you don't have a friend or family member with T-Mobile and the same size SIM your device requires, you can go to a T-Mobile store and they can help troubleshoot.