I am sorry to hear that your Gmail is not displaying emails since April 1st, and can understand the disruption that can cause. As Communityuser_80 pointed out, making sure your sync settings are on can help correct the problem. Another helpful thing to do is make sure you have the data to load your emails, whether that's mobile network or Wi-Fi, so check your connections to see if that is playing a part, and check your Gmail on another device (such as computer) to see if it is just on your phone or if it is your Gmail/Google account in general.
You can try clearing the cache and data on your Gmail application by going to Settings > Apps >find Gmail > App Info > Storage, and tapping to clear your cache and data. You may have to sign back in after doing this.
If that does not work, you can try Safe Mode to see if a third-party application may be interfering. The guide for Safe Mode: https://www.samsung.com/us/support/answer/ANS00062983/
If that does not help, your Gmail/Google account has no issues on other devices, and your connection(s) are sufficient, it may be time to get your device evaluated. You can view your options here: https://samsung.com/us/support/service