Turn the phone off. Wait a few seconds. Turn it back on.
If that doesn't bring the pictures back, turn it off again and remove the SD card, then turn it on. (Gallery shows what Media Scanner finds when it scans the phone, which happens when you turn the phone on or restart it, and is supposed to happen any time you make a media change, like saving a picture. If the SD card is bad, Media Scanner can get stuck on the SD card and never find any files.)
If that doesn't do it, the files got deleted. It's possible to get them back, but you have to root the phone (you can't recover deleted files unless the phone is rooted) and install an app like
DiskDigger (the free version recovers pictures only). Rooting and installing an app can overwrite one or more of the pictures you're trying to recover, but it's the best there is.
In the future, copy any file you don't want to lose to another device (laptop, tablet, desktop)
and a cloud account or two. ("Too many backups" is just words without meaning - you can't have too many.) Devices
do go bad. If your phone, your laptop, Dropbox and Google Drive all fail at the same moment, you have a bigger problem than pictures - like a planet to live on, because this one will have disappeared.
Also, if the phone is under warranty, rooting voids the warranty, so I'd take the phone to the carrier first. If they root it to get your pictures back, they don't void their own warranty. (Although they'll probably tell you that there's nothing they can do, but it's worth asking if they can 1) get your pictures back and 2) fix whatever went wrong in the phone.)
If th pictures had been on the SD card, there's a simple way to get them back -
PhotoRec, a free program worth
my weight in gold (and I'm not thin).