Did you restart your device?
Power down and remove the battery and power on?
Power down and removing the sim card and reinstall?
Turn airplane mode ON for a minute or two and then turn it OFF?
If none of those work you'll need to go to a Verizon corporate store and get a free new SIM card.
I am also having issues with my Verizon wireless signal on my galaxy s5. It goes from 3g to 1x to to no service all the time. I've missed important calls from my school and friends because of this issue. I've done research on this problem and I know I'm not the only one. It seems it's only the Verizon customers. So I know it can't be the phone itself. I've done the battery pull and Sim card pull multiple times. It doesn't work. I've also learned verizon customer service is no help. So let's not waste time with that. And I've read post on this same exact issues from 2011. It's 2015 now. So the issue is not fixed yet. The next update probably won't solve it either (not that I'm willing to wait either). So if anyone with this problem found a fix, please let me know. This sh*t is corny. I would switch providers but I have to wait like 2 years to be released from contract.