This is only if you have messed with spawn settings at all.

Tornadoes can have long periods of spawning on default settings, that is intentional design. But it is possible to break mod if you put the timer really low and spawn chances super high.
Easiest way to fix this is to type

gautospawn_timer 120
gautospawn_spawn_chance 3

Then you must quit the map and restart gmod. If your game crashes it will not save your changes.
If you cannot do this then follow these directions

Go to C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\cfg or wherever you have gmod installed.
Fnd server.vdf open it in notepad and find anything with gautospawn_ and just delete those lines. That should hard reset the setting
or delete the whole file. (Not recommended as this WILL reset all other settings not related to the autospawner)