This morning I woke up to my phone saying "Insert SIM card" but my SIM is in the phone!

I tried another working SIM in the phone and it said the same thing and I tried my SIM in another phone and it worked fine so I think its safe to say its the phone thats up, but I don't know whats up with it or how (or if it can be) to fix it.

Any ideas?

Cheers,
David