ive got the same prob on my 4s i usually have to hard reboot a couple tims with no sim
steps i go through
1. turn on airplane mode
take out sim
2. hard reboot
3.turn back on then turn off airplane a couple a times if didnt work 1st time
another way that worked for me is turn on airplane mode then reset network settings and reboot with out sim in
but the longest this worked for me was maybe 3 or 4 days it a temp fix.. it just went to searching again about 5 mins ago and i used my first way to fix it, get it to say no sim and then i put sim back in. note it has to say no sim when its on the way to be working