Take a deep breath.
I meant it! Exit game, reboot computer and try again.
run Clean Installer and make sure you have no duplicates (pink highlighted files) and no packages marked "empty package" in your Downloads.
Navigate to your Sims 2 folder in My Documents and delete the files groups.cache cigen.cache and accessories.cache (they will be replaced on next loading the game. (Since installing Free Time I seem to have to do this almost every time I run the game, or BodyShop, or HomeCrafter plus). NeighbourhoodManager.package can also become corrupt and is safe to delete. These files can become corrupted stopping your game from loading. Its safe to delete as the game will generate a new one on loading. More information about this.
Delete the contents of the Thumbnails folder. These too will be regenerated.
Defrag your computer (if your game is installed on a drive other than C, defrag both C and the drive the game is on.)
Make sure there is plenty of space on BOTH these drives.
It may be helpful to remove the contents of the SavedSims folder.
If the game will load as far as the Neighbourhood Screen, enter boolprop testingcheatsenabled true - and then try to enter the suspect lot. Sometimes this will give you information as to what object causes the crash.It is generally safe to delete the problem object it picks up, if it does..
If you do have the CEP installed, you might want to try removing the _EnableColorOptionsGMND.package file before loading your game - temporary fix only, and don't save the lots if they DO load or your recoloured items will disappear... this is because if _EnableColorOptionsGMND.package file has become corrupted, the game will crash whenever you try to enter a lot with an object that is listed in the _EnableColorOptionsGMND.package. A bad GMND resource will crash the game. If the suspect lots all load with the CEP removed, uninstall and reinstall the CEP.
It may help to turn off antivirus software.
Increase paging memory size. FOR XP: Right click on the "My Computer" icon from your desktop, then choose "Properties" and then the "Advanced" tab -at the top, it should say Performance - visual effects, processor scheduling, memory usage, and virtual memory - click "Settings" next to that = then click "Advanced" tab again, and "Change" next to virtual memory.
Remove Maxis objects (downloaded from Store etc....) this is a personal thing but I have found many times that an EA/Maxis item (which usually show up in red as Game Objects in Clean Installer) can be the culprit. ALSO as Sims 3 items are also called .package files, be careful that you have no Sims 3 downloads in there... I have had this cause crashes. This may be because I move download files around a lot and put some in the wrong folder. Uninstalling Sims 3 and not downloading stuff for it helps!
Check for .txt, .doc, .html, .jpg, .png, .bmp, .Sims2Pack etc files - whatever you can think of that might have landed in the Downloads folder by mistake or misclick - also .rar and .zip of course. Clean Installer won't find these, Windows Search will or a visual inspection of the files will (sort by type).
Hack Conflict Detector may pick up problems with user-made hacks.
Clean dust out of your computer!
Remove all hack files. (I keep folders called Hacks and HackedObjects which contain, in subfolders, all my hacked items - much easy to remove this way.)
Remove any custom careers or custom University Majors - AFTER checking that any Sims using these are at home and not at work or you might lose those Sims. (A good policy is not to save while a member of the family is at work, wait till they are all at home).
If your computer crashed during a SAVE, it may have corrupted files in the game files in My Documents/EA Games/The Sims 2. You can simply rename that folder and let the game create a new one. You can even attempt to salvage your old 'hoods and Downloads if you like once you have a functional game again. Miros says that making all your game files (the package files in C:/Program Files/EA Games/The Sims) read-only helps prevent this.
If that still does not work, remove all Downloads: if the game loads properly without downloads then you know it is a Download. Isolate the problem file by
the binary method - take half the downloads and put them back in, try again, take half of what remains and return that... sooner or later you will find a chunk that contains the problem download.
There is a very nice explanation of the Binary method at Mod the Sims 2, complete with diagram for the dyslexics among us!
The way I personally do this is NOT the binary method - I do a folder at a time, as my downloads are sorted by folder, and I usually have a fair idea of what I added recently. This CAN be slower but it also allows for weeding of content "Wow, I didn't realise I still had THIS installed...I've never used it and am not likely to."
Delphy made a program that isolates orphaned files. I personally haven't found it that reliable, but it might help. I don't think orphaned meshes alone will crash the game but hygiene is always good, and what's the point of having files there that you never use?
If the game still doesn't load reliably, both Neighbourhoods and Lots, THEN it's time to reinstall.
Once reinstalled, you may well be able to get your
Neighbourhoods back, IF you saved out the Neighbourhood files before
uninstalling. Of course unless you also put back the same custom content the
Sims may well look different.