That Mrs. Crumplebottom can be quite nasty when you're Sims are displaying signs of affection out in the open is a widely-known fact. However, when she is kissed herself, by a vampire that is, she becomes so mean she corrupts the game. The game corrupts itself anyway. When Mrs. Crumplebottom is turned into a vampire, a bug in the code of Nightlife corrupts the crucial 'objects.package' file, which contains career icons and other important data. When it's broken, the game will not seem to be working very well anymore. The quick fix is to not let Mrs. Crumplebottom be kissed by a vampire.

If she has been changed into a vampire, the best is to replace the objects.package file. On CD 2 of Nightlife, there should be a file called 'compressed.zip'. Open this with a Zip program (e.g. WinZip), and extract the file 'objects.package' from the zip file, and place it in C:\Program Files\EA Games\The Sims 2 Nightlife\TSData\Res\Objects. Overwrite the existing file if asked.

To prevent the file from becoming corrupted, browse to the "C:\Program Files\EA Games\The Sims 2 Nightlife\TSData\Res\Objects" folder. There, select the objects.package file (don't open it!), and then choose Properties in the File menu (at the top). Then tick the "Read-only" checkbox, and click OK. This will make the file Read-only, and thus the game cannot write to it, corrupting it in the process.

A Maxoid has already said that the bug will probably not be fixed in the first Nightlife patch, that's very near to its deadline now and will be released soon. A second patch will probably be made to fix the problem. More details are in this thread at Mod The Sims 2. Thanks to SimsZone and SP Zone for the link.