In Colorado Springs, NBC Channel 5.1 went through a repacking, which is a change of the broadcast frequency. While it will still show up as channel 5.1, the actual frequency changed from 42 to 25.
Here’s more info about it: https://www.koaa.com/about-us/watch-koaa-over-the-air-youll-need-to-rescan-your-tv-on-february-27?utm_source=website&utm_medium=local-promo-right-rail&utm_campaign=redesign
All it means for you is you need to do a re-scan, and NBC will be back on channel 5.1
To scan for channels on a Fire TV device:
1) Select Settings, and then Live TV.
2) Then, select Live TV Sources > Fire TV Recast > Channel Scan
If you do not have a FireTV device, this video might be useful to you. Watch KOAA over the air? You’ll need to rescan your TV on February 27KOAA