In playing Half-Life 2 you may run into the problem that you get the message 'Node Graph Out Of Date. Rebuilding...' and from that point the AI may cease to function.

There are 2 ways that this can happen:
If the message appears when a map is first loaded, and there is a period where the screen is frozen and sound stutters (i.e. the game is doing something in the background), then it actually is rebuilding the graph (which is what the AI uses to find its way around), and everything should be fine.

However, if the message appears but the game does no processing, i.e. the game does not freeze when the message appears (most commonly happens when loading a savedgame), then the graph has not been rebuilt, and the AI will not function.

There are actually 2 distinct problems here; the lack of a node graph and the AI being switched off. The AI can be toggled in the console by running the ai_disable command, this will solve the AI being switched off. If the AI is switched off nothing will move. This is not a solution to the graph problem however, as while basic AI like the zombies are fine, more advanced AI, like the antlions following you, breaks without the graph.

To rebuild the node graph, the map must be loaded from scratch. This is easily done with the 'map' command. First, start the game and load your save. Assuming the graph is not correctly rebuilt, go into the console and look for the last line which starts 'spawnserver'. Right after this is the name of the map that was last loaded, which is the map you are currently on. Type 'map' followed by this name and hit return to load the map from scratch, which should cause the graph to be rebuilt, you'll know because it will say it is doing it, and will freeze for a few seconds as described above. Once this is done, load your game again and it should be back to normal. If the AI is still switched off run 'ai_disable' to toggle it back on.