It will not open
A crash on the icon, or a flash of the home screen and a close, is a crash loop. Restart the phone once. If it still closes, undo the last theme or font if you can reach settings for a second. Do not clear data yet.
The step-by-step for that loop is GB WhatsApp Pro that keeps closing after you open it. Roll back to the previous build only after you know whether a backup exists.
It opens, but messages do not move
Chats that load while new texts stay on the clock are a send problem, not a failed install. Check date and time, turn off any VPN, and try mobile data. If that started the same day as the update, follow messages stuck on the clock in GB WhatsApp Pro before you uninstall.
A login loop that keeps asking for a code is different again. Read the banner. If it says to use official WhatsApp, more SMS codes will not finish the update.
A menu you used is gone
Some releases move GB settings under a renamed item or drop an experimental switch. Search settings for “GB” before you assume the feature was deleted. If you installed official WhatsApp by mistake, the mod menus will be absent because that is a different app. Confirm the name on the About screen matches the build you meant to update.