See if the SIM works in another phone first. SIMs do die on occasion.
Also check your SIM settings in Settings -> mobile + SIM
If it's neither of those things you may have to consider some other options.
Excellent advice. Sometimes just removing and reinstalling the SIM corrects problems. And having two 925's, I think their SIM tray and contact system is inferior.
I bought two 925's used on eBay. They were obviously some kind of new, since they came in factory boxes and with all the pieces of paper, and a SIM card.
Yet, neither had cellular nor wifi connectivity as good as any phone I've had. I sent one off to Nokia for warranty repair, it came back as good as anything. Well, actually it was a different IMEI. Same end result. I have the other one in repair now for the same issue.
The most simple and most likely reason for bad connectivity is corrosion in the internal antenna cable (there are THREE! antennas in the 925!) Some years ago I had a Nokia 9290 that over time just became less and less sensitive. I sent it in for warranty repair, they told me that it had water damage and it would cost me a bundle to fix it. almost as much as the phone cost! I screamed bloody murder because I knew it have never been closer to water than my shirt pocket. They backed off and fixed it. The final paperwork, reading between the lines, said that they cleaned the antenna cable contacts. I've been in electronics for half a century, so I know that means they just wiggled/disconnected/reconnected the suspect connection points, and/or sprayed old fashioned contact cleaner on them.
If your phone is out of warranty and you are willing to open it up, I posted the service manual here awhile back. If you need it, ask.