Depends what game it is, but all games may have lag solved by a restart.

Minecraft:
Most likely a restart will fix this, but you can optimize your server for this to not happen. You can lower spawnrate, maxspawns, etc.

Unturned:
What exactly is the lag?
FPS?- This is very likely to be caused by an unoptimized map, lots of graphic processing like shaders(/lights) and such. Try optimize map to fix this.
Ping?- This is usually caused by internet connection. It can be caused by low TPS/FPS because server<->client delays communicating.
Players bug/teleport- This is almost always caused by anticheat, you can solve it by fixing the map, or you can disable the playermovementcheck in Server/Execute.config.