Messenger app, created by Google is currently one of the most popular SMS apps. App was downloaded around 100 million times, and that's not the end because Google also pre-installed it on their phones from Nexus series, Pixel series and phones from Android One project. Yesterday I noticed that my friend sent an SMS to me, but I haven't received any notification about it. I searched for info about this problem, and how it turned out - I wasn't the only person with a similar issue.
Whole problem is caused by new update of Messenger to 2.5.207 version. Google said that firm is working on a solution of this problem, but company haven't mentioned deadline when repaired version of app will be available in Google Play Store. Until then, if you have pre-installed Messenger on your phone, you just have to wait for new version or you can download other SMS app. If you downloaded Messenger manually on your phone, you can downgrade Messenger to previous version by downloading proper .apk file from the Internet, because on older versions problem doesn't exist.