I recently had an issue with my Drupal installation. After I run the update.php script my installation crashed. I only installed a few modules but then that should not have crashed it as the modules run fine on my other websites. Being an optimistic person I did not have a backup of my database to fall back on. I was totally royally screwed as that was a website for school and it should not have gone down at any cost.
It took me some time to get back to my senses and think calmly as to why Drupal will behave so erratically. I tried running the update.php script again but the error would not go away. I tried deleting the module and updating but even that wouldn't help.
Then I looked into the settings.php that resides in sites/default folder. I changed my memory limit for php by changing the line ini_set('memory_limit', '32M'); to have 64 Megabytes of memory for my installation. I ran update.php again and the website worked fine.
Here, I would like to tell you that the update.php script was running fine every time I tried it but just that it wouldn't let me access my home page or any other admin page for that matter. Hope this will help someone.