Unfortunately there isn't a lot you can do. There are a couple reasons the app may not be showing - either it's been removed or it's not compatible with your device anymore.
If it's been removed, there isn't much you can do but find an alternate place to get the apk, but be careful about the alternate sources as they can bring you other issues.
If it's not compatible with your device, there's a reason so getting it from another source may not be of any benefit.